fix interop and indexing oversights

TODO.md

@@ -603,53 +603,22 @@
   - structurally identical anonymous struct payloads share type identity, so sum composition merges
     matching variants and still rejects same-name variants with different payload shapes
 
-24. bug fixes & interop/indexing oversights (surfaced by the raylib testbed)
+24. bug fixes & interop/indexing oversights (implemented)
-  - the bouncing-shapes testbed (`testbed/game`) drove most of the language at once and
+  - `return match ...` and `yield match ...` are accepted as direct value-control-flow
-    exposed several gaps; grouped here as the next polish pass. it currently compiles only by
+    operands, matching declaration/assignment value sources
-    keeping three non-interconverting integer "worlds" (`usize` for indices, the `int`
+  - index and slice-bound expressions are contextually coerced to `usize`; unsigned narrower
-    constraint for `c_int` args, `c_float` for physics) and routing around the items below
+    integer indices work, while signed runtime indices diagnose instead of reaching LLVM
-  - `return match ...` doesn't work (and `yield match ...` probably doesn't either): a `match`
+  - concrete native scalars coerce to same-family C scalar types at call, return, assignment,
-    is not accepted directly as a `return`/`yield` operand, so the value must be bound to a
+    aggregate, and optional boundaries when the target C type can represent the source width
-    local first, or a statement-`match` with per-arm `return` used — the testbed's `next_kind`
+  - scalar keyword casts (`i32(x)`, `usize(x)`, `c_float(x)`, etc.) provide explicit numeric
-    had to do the latter
+    conversions for cases that should not be implicit
-  - signed-index array access must be a diagnostic, not a miscompile: `arr[i]` with a signed
+  - C scalar comparisons accept numeric literals by typing the literal from the concrete C
-    index (`i32`/`int`, including the `int` constraint) currently lowers to malformed LLVM IR
+    operand, so aliases like `ZF` are no longer needed
-    (the indexed slot's address and the stored value get swapped: `store <struct>, ptr` with a
+  - array counts accept compile-time integer expressions such as `[CAP]T` and `[N + 1]T`;
-    value where a `ptr` is expected) for both reads and writes; only `usize` and compile-time
+    runtime variables remain rejected because arrays are fixed-size values
-    literal indices work. settle the rule — index expressions require an unsigned
+  - string literals in value-`match` / value-`if` peers resolve to a common zero-terminated
-    (`usize`-coercible) type, as in Rust/Zig — and report a clear checker error instead of
+    byte slice when possible, and `[;0]u8` slices can decay to immutable `*c_char`/`?*c_char`
-    emitting bad IR
+    parameters
-  - native scalar types should coerce to their C equivalents in general: `f32 -> c_float`,
-    `f64 -> c_double`, and the integer cases (`i32 -> c_int`, `u8 -> c_uchar`,
-    `usize -> the matching C width`, …) at call/return/assignment boundaries, following C's own
-    same-width/family conversions. today only the `int` constraint coerces to `c_int`;
-    concrete-width native scalars (`i32`, `usize`, `f32`) are stranded, which is why the testbed
-    had to store physics directly in `c_float` and maintain parallel `usize`/`int` counters
-  - explicit scalar type casting: there is no cast syntax yet (`c_float(x)` is rejected — that
-    spelling is distinct-type construction, not a numeric cast), so any conversion the coercion
-    rules above don't cover is currently impossible. settle a cast spelling; this removes most
-    of the manual world-juggling the testbed needed
-  - c scalars must be comparable with numeric literals: `x < 0.0` and `x != 0.0` where
-    `x : c_float` currently error ("comparison requires compatible numeric operands") even
-    though `x + 1.0` / `x * 2.0` already accept the literal. a comparison operand literal should
-    adopt the other operand's concrete (c) scalar type exactly as arithmetic does — otherwise
-    every zero/bound needs a `c_float` constant alias (the testbed added `ZF`, `SPINMAX`, etc.)
-  - array sizes should accept compile-time-constant expressions, not just integer literals:
-    `[CAP]T` with `CAP :: 64` (and `[N + 1]T`, …) should resolve through the same constant
-    folding used for global initializers. runtime *variable* lengths stay rejected by design —
-    a runtime-length array is a slice + allocation (milestone 25), not an array type (Zig/Rust
-    parity: array lengths are comptime-known). the testbed had to hard-code `[64]` and keep a
-    separate `CAP usize :: 64` for the bounds checks
-  - peer-type resolution for string literals in value-`match` / value-`if` arms (Zig parity,
-    not a brolang-specific limitation): arms yielding differently-sized string literals
-    (`@[6;0]u8` for "circle" vs `@[8;0]u8` for "triangle") currently fail to unify, forcing a
-    per-arm `DrawText("…")` workaround instead of `tag :: match k { … }; DrawText(tag, …)`.
-    verified Zig 0.16 behavior: `switch`/`if` arms of string literals peer-resolve to a single
-    sentinel slice `[:0]const u8` (preserving the common `:0`), which then coerces directly to a
-    C string `[*c]const u8`. brolang should (a) peer-type-resolve same-sentinel string literals
-    to a sentinel byte slice `[;0]u8`, and (b) allow a zero-terminated byte slice to convert to
-    `*c_char` (extending milestone 3.5's pointer-view → `*c_char` rule to the matching sentinel
-    slice). with both, a value-`match` label passes straight to a `?*c_char` parameter
 
 25. dynamic heap allocation
   - see below for direction

compiler/ast/ast.odin

@@ -83,6 +83,7 @@ Expr_Kind :: enum u8 {
 	Orelse,
 	Struct_Literal,
 	Keyed,
+	Cast,
 	Negate,
 	Not,
 	Add,
@@ -109,6 +110,7 @@ Expr :: struct {
 	args:       []Expr_Id,
 	qualifier:  symbol.Id,
 	name:       symbol.Id,
+	type:       Type_Syntax,
 	left:       Expr_Id,
 	right:      Expr_Id,
 	body:       []Stmt_Id,

compiler/checker/checker.odin

@@ -296,6 +296,73 @@ eval_constant :: proc(checker: ^Checker, expr_id: ast.Expr_Id) -> Constant {
 	return checker.constants[expr_id]
 }
 
+eval_integer_constant_in_context :: proc(
+	checker: ^Checker,
+	expr_id: ast.Expr_Id,
+	pkg: ast.Package_Id,
+	file: ast.File_Id,
+	depth := 0,
+) -> Constant {
+	if depth > 64 || expr_id == ast.INVALID_EXPR || int(expr_id) >= len(checker.ast_module.exprs) {
+		return Constant{kind = .Not_Constant}
+	}
+	expr := checker.ast_module.exprs[expr_id]
+	#partial switch expr.kind {
+	case .Integer:
+		return Constant{kind = .Value, value = i128(expr.integer)}
+	case .Name:
+		target_pkg, available := expr_package(checker, expr, pkg, file, false)
+		if !available {
+			return Constant{kind = .Not_Constant}
+		}
+		global := find_global(checker, expr.name, target_pkg)
+		if global == ast.INVALID_GLOBAL || int(global) >= len(checker.ast_module.globals) {
+			return Constant{kind = .Not_Constant}
+		}
+		g := checker.ast_module.globals[global]
+		if g.external || !g.immutable {
+			return Constant{kind = .Not_Constant}
+		}
+		return eval_integer_constant_in_context(checker, g.expr, g.pkg, g.file, depth+1)
+	case .Negate:
+		operand := eval_integer_constant_in_context(checker, expr.left, pkg, file, depth+1)
+		if operand.kind == .Value {
+			value, overflow := intrinsics.overflow_sub(i128(0), operand.value)
+			return Constant{kind = .Overflow} if overflow else Constant{kind = .Value, value = value}
+		}
+		return operand
+	case .Add, .Sub, .Mul, .Div:
+		left := eval_integer_constant_in_context(checker, expr.left, pkg, file, depth+1)
+		right := eval_integer_constant_in_context(checker, expr.right, pkg, file, depth+1)
+		if left.kind == .Div_By_Zero || right.kind == .Div_By_Zero {
+			return Constant{kind = .Div_By_Zero}
+		}
+		if left.kind == .Overflow || right.kind == .Overflow {
+			return Constant{kind = .Overflow}
+		}
+		if left.kind != .Value || right.kind != .Value {
+			return Constant{kind = .Not_Constant}
+		}
+		value: i128
+		overflow: bool
+		#partial switch expr.kind {
+		case .Sub:
+			value, overflow = intrinsics.overflow_sub(left.value, right.value)
+		case .Mul:
+			value, overflow = intrinsics.overflow_mul(left.value, right.value)
+		case .Div:
+			if right.value == 0 {
+				return Constant{kind = .Div_By_Zero}
+			}
+			value = left.value / right.value
+		case:
+			value, overflow = intrinsics.overflow_add(left.value, right.value)
+		}
+		return Constant{kind = .Overflow} if overflow else Constant{kind = .Value, value = value}
+	}
+	return Constant{kind = .Not_Constant}
+}
+
 fits_signed_type :: proc(value: i128, value_type: types.Type, selected := target.DEFAULT) -> bool {
 	if !types.is_signed(value_type, selected) {
 		return false
@@ -320,14 +387,83 @@ fits_i64 :: proc(value: i128) -> bool {
 	return fits_signed_type(value, types.I64)
 }
 
-type_from_syntax :: proc(value: ast.Type_Syntax) -> types.Type {
+type_from_syntax :: proc(
+	checker: ^Checker,
+	value: ast.Type_Syntax,
+	pkg := ast.Package_Id(0),
+	file := ast.File_Id(0),
+	depth := 0,
+) -> types.Type {
+	if depth > 64 {
+		return types.INVALID
+	}
+	item, ok := types.node(&checker.module.types, value)
+	if !ok {
+		return value
+	}
+	store := &checker.module.types
+	changed := false
+	#partial switch item.kind {
+	case .Array:
+		child := type_from_syntax(checker, item.child, pkg, file, depth+1)
+		changed = changed || child != item.child
+		item.child = child
+		if item.unresolved_count {
+			expr_id := ast.Expr_Id(item.count_expr)
+			span := source.Span{}
+			if expr_id != ast.INVALID_EXPR && int(expr_id) < len(checker.ast_module.exprs) {
+				span = checker.ast_module.exprs[expr_id].span
+			}
+			constant := eval_integer_constant_in_context(checker, expr_id, pkg, file)
+			if constant.kind == .Value {
+				switch {
+				case constant.value < 0:
+					source.add(checker.diagnostics, span, "array count must be non-negative")
+					return types.INVALID
+				case constant.value > i128(0xffff_ffff_ffff_ffff):
+					source.add(checker.diagnostics, span, "array count does not fit in u64")
+					return types.INVALID
+				case:
+					item.count = u64(constant.value)
+					item.unresolved_count = false
+					item.count_expr = 0
+					changed = true
+				}
+			} else {
+				source.add(checker.diagnostics, span, "array count must be a compile-time integer expression")
+				return types.INVALID
+			}
+		}
+	case .Pointer, .Slice, .Optional, .Range, .Distinct, .Enum, .Fallible:
+		child := type_from_syntax(checker, item.child, pkg, file, depth+1)
+		extra := type_from_syntax(checker, item.extra, pkg, file, depth+1)
+		changed = child != item.child || extra != item.extra
+		item.child = child
+		item.extra = extra
+	case .Function:
+		params := types.params_for(store, value)
+		resolved_params := make([]types.Type, len(params), checker.allocator)
+		defer delete(resolved_params, checker.allocator)
+		params_changed := false
+		for param, index in params {
+			resolved_params[index] = type_from_syntax(checker, param.type, pkg, file, depth+1)
+			params_changed = params_changed || resolved_params[index] != param.type
+		}
+		result := type_from_syntax(checker, item.child, pkg, file, depth+1)
+		if params_changed || result != item.child {
+			return types.function(store, resolved_params, result, item.c_abi, item.variadic)
+		}
+	}
+	if changed {
+		return types.intern(store, item)
+	}
 	return value
 }
 
 function_channel_type :: proc(checker: ^Checker, function: ast.Function) -> types.Type {
-	result := type_from_syntax(function.result)
+	result := type_from_syntax(checker, function.result, function.pkg, function.file)
 	if types.is_valid(function.error) {
-		return types.fallible(&checker.module.types, result, type_from_syntax(function.error))
+		return types.fallible(&checker.module.types, result, type_from_syntax(checker, function.error, function.pkg, function.file))
 	}
 	return result
 }
@@ -642,11 +778,11 @@ valid_call_arity :: proc(function: ast.Function, count: int) -> bool {
 	return count >= len(function.params) if function.variadic else count == len(function.params)
 }
 
-call_arg_expected :: proc(function: ast.Function, index: int) -> types.Type {
+call_arg_expected :: proc(checker: ^Checker, function: ast.Function, index: int) -> types.Type {
 	if index < 0 || index >= len(function.params) {
 		return types.INVALID
 	}
-	declared := type_from_syntax(function.params[index].type)
+	declared := type_from_syntax(checker, function.params[index].type, function.pkg, function.file)
 	// A `float` param defaults to f64 so an integer-literal argument builds as a
 	// float constant (e.g. `f(3)` -> 3.0), mirroring `pi float = 3` for locals.
 	// `int`/`range` constraints have no single default and keep building naturally.
@@ -682,13 +818,13 @@ function_value_signature :: proc(
 	if !function.c_abi || types.is_valid(function.error) {
 		return nil, types.INVALID, false
 	}
-	result = type_from_syntax(function.result)
+	result = type_from_syntax(checker, function.result, function.pkg, function.file)
 	if !types.is_void(result) && !is_runtime_type(checker, result) {
 		return nil, types.INVALID, false
 	}
 	params = make([]types.Type, len(function.params), checker.allocator)
 	for param, index in function.params {
-		param_type := type_from_syntax(param.type)
+		param_type := type_from_syntax(checker, param.type, function.pkg, function.file)
 		if !is_runtime_type(checker, param_type) {
 			delete(params, checker.allocator)
 			return nil, types.INVALID, false
@@ -758,7 +894,7 @@ mark_expr_imports_used :: proc(checker: ^Checker, expr_id: ast.Expr_Id, file: as
 			if expr.left != ast.INVALID_EXPR {
 				append(&stack, expr.left)
 			}
-		case .Negate, .Not, .Address, .Deref, .Field, .Unwrap, .Try, .Keyed, .Enum_Literal:
+		case .Negate, .Not, .Address, .Deref, .Field, .Unwrap, .Try, .Keyed, .Enum_Literal, .Cast:
 			append(&stack, expr.left)
 		case .Catch:
 			append(&stack, expr.left)
@@ -894,7 +1030,7 @@ validate_declarations :: proc(checker: ^Checker) {
 		locals: [dynamic]symbol.Id
 		locals.allocator = checker.allocator
 		for param in function.params {
-			if diagnostic := add_unsupported_type_diagnostic(checker, param.span, type_from_syntax(param.type));
+			if diagnostic := add_unsupported_type_diagnostic(checker, param.span, type_from_syntax(checker, param.type, function.pkg, function.file));
 			   diagnostic != source.INVALID_DIAGNOSTIC {
 				checker.template_diagnostics[function_id] = diagnostic
 				continue
@@ -915,7 +1051,7 @@ validate_declarations :: proc(checker: ^Checker) {
 				)
 			}
 			append(&locals, param.name)
-			if types.contains_c_struct_by_value(type_from_syntax(param.type), &checker.module.types) {
+			if types.contains_c_struct_by_value(type_from_syntax(checker, param.type, function.pkg, function.file), &checker.module.types) {
 				checker.template_diagnostics[function_id] = source.addf(
 					checker.diagnostics,
 					param.span,
@@ -924,11 +1060,11 @@ validate_declarations :: proc(checker: ^Checker) {
 				)
 			}
 		}
-		if diagnostic := add_unsupported_type_diagnostic(checker, function.span, type_from_syntax(function.result));
+		if diagnostic := add_unsupported_type_diagnostic(checker, function.span, type_from_syntax(checker, function.result, function.pkg, function.file));
 		   diagnostic != source.INVALID_DIAGNOSTIC {
 			checker.template_diagnostics[function_id] = diagnostic
 		}
-		if types.contains_c_struct_by_value(type_from_syntax(function.result), &checker.module.types) {
+		if types.contains_c_struct_by_value(type_from_syntax(checker, function.result, function.pkg, function.file), &checker.module.types) {
 			checker.template_diagnostics[function_id] = source.addf(
 				checker.diagnostics,
 				function.span,
@@ -937,7 +1073,7 @@ validate_declarations :: proc(checker: ^Checker) {
 			)
 		}
 		if types.is_valid(function.error) {
-			error_type := type_from_syntax(function.error)
+			error_type := type_from_syntax(checker, function.error, function.pkg, function.file)
 			error_sum := types.is_enum(error_type, &checker.module.types) ||
 			             types.is_tagged_union(error_type, &checker.module.types)
 			if function.c_abi {
@@ -972,7 +1108,7 @@ validate_declarations :: proc(checker: ^Checker) {
 		}
 		if !function.has_body && function.c_abi {
 			for param in function.params {
-				param_type := type_from_syntax(param.type)
+				param_type := type_from_syntax(checker, param.type, function.pkg, function.file)
 				if add_unsupported_type_diagnostic(checker, param.span, param_type) !=
 				   source.INVALID_DIAGNOSTIC {
 					continue
@@ -989,7 +1125,7 @@ validate_declarations :: proc(checker: ^Checker) {
 					)
 				}
 			}
-			result := type_from_syntax(function.result)
+			result := type_from_syntax(checker, function.result, function.pkg, function.file)
 			if add_unsupported_type_diagnostic(checker, function.span, result) == source.INVALID_DIAGNOSTIC &&
 			   !types.contains_c_struct_by_value(result, &checker.module.types) &&
 			   !types.is_c_signature_type(result, &checker.module.types, true) {
@@ -1174,7 +1310,7 @@ find_spec :: proc(checker: ^Checker, template: ast.Function_Id, actual_args: []t
 			if param_index < len(actual_args) {
 				actual = actual_args[param_index]
 			}
-			if !types.equal(spec.args[param_index], specialized_param_type(checker, param.type, actual)) {
+			if !types.equal(spec.args[param_index], specialized_param_type(checker, param.type, actual, function.pkg, function.file)) {
 				matches = false
 				break
 			}
@@ -1190,8 +1326,14 @@ find_spec :: proc(checker: ^Checker, template: ast.Function_Id, actual_args: []t
 // type for a given actual argument. A constraint param (`int`/`float`/`range`)
 // resolves to the actual's family member (INVALID if out of family), so a call
 // passing an out-of-family argument fails to specialize and is rejected.
-specialized_param_type :: proc(checker: ^Checker, syntax: ast.Type_Syntax, actual: types.Type) -> types.Type {
+specialized_param_type :: proc(
-	declared := type_from_syntax(syntax)
+	checker: ^Checker,
+	syntax: ast.Type_Syntax,
+	actual: types.Type,
+	pkg: ast.Package_Id,
+	file: ast.File_Id,
+) -> types.Type {
+	declared := type_from_syntax(checker, syntax, pkg, file)
 	if types.is_constraint(declared) {
 		return types.constraint_target(declared, actual, &checker.module.types)
 	}
@@ -1204,7 +1346,7 @@ can_specialize :: proc(checker: ^Checker, function: ast.Function, actual_args: [
 		if index < len(actual_args) {
 			actual = actual_args[index]
 		}
-		if !is_runtime_type(checker, specialized_param_type(checker, param.type, actual)) {
+		if !is_runtime_type(checker, specialized_param_type(checker, param.type, actual, function.pkg, function.file)) {
 			return false
 		}
 	}
@@ -1223,7 +1365,7 @@ ensure_spec :: proc(checker: ^Checker, template: ast.Function_Id, actual_args: [
 		if index < len(actual_args) {
 			actual = actual_args[index]
 		}
-		append(&signature, specialized_param_type(checker, param.type, actual))
+		append(&signature, specialized_param_type(checker, param.type, actual, function.pkg, function.file))
 	}
 	result := function_channel_type(checker, function)
 	if function.pkg == 0 && function.name == checker.main_symbol && function.result == types.INT &&
@@ -1335,6 +1477,9 @@ infer_compound_expr :: proc(
 			_ = infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
 		}
 		return types.INVALID
+	case .Cast:
+		_ = infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
+		return type_from_syntax(checker, expr.type, pkg, file)
 	case .Address:
 		child := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
 		return types.pointer(store, child, false, false)
@@ -1487,7 +1632,7 @@ infer_expr :: proc(
 				last = types.F64
 				_ = pop(&stack)
 			case .String, .Array, .None, .Undefined, .Address, .Deref, .Index, .Slice,
-			     .Field, .Unwrap, .Orelse, .Try, .Catch, .Struct_Literal, .Keyed, .Enum_Literal,
+			     .Field, .Unwrap, .Orelse, .Try, .Catch, .Struct_Literal, .Keyed, .Enum_Literal, .Cast,
 			     .Bool, .Not, .Eq, .Ne, .Lt, .Le, .Gt, .Ge, .And, .Or, .Range:
 				last = infer_compound_expr(checker, expr, locals, pkg, file, demanded, local_types)
 				_ = pop(&stack)
@@ -1701,7 +1846,7 @@ infer_expr :: proc(
 			// it (e.g. `take_u16(a)` resolves `a` to u16). Constraint params have no single
 			// type to demand; the callee's result flowing back is milestone 14.5.
 			for arg_index in 0..<len(expr.args) {
-				record_demand(checker, expr.args[arg_index], call_arg_expected(function, arg_index), locals, local_types, pkg, file)
+				record_demand(checker, expr.args[arg_index], call_arg_expected(checker, function, arg_index), locals, local_types, pkg, file)
 			}
 			// Deferred defaulting leaves an undemanded open constant typeless; give such an
 			// argument its default so the call can still monomorphize (the default feeds only
@@ -1865,7 +2010,7 @@ infer_statements :: proc(
 				// binding (its declared type when annotated, else left open) and walk
 				// the block body. The build pass resolves the yielded value's type
 				// independently — value blocks don't join the demand fixpoint.
-				declared_block := type_from_syntax(statement.type)
+				declared_block := type_from_syntax(checker, statement.type, pkg, file)
 				block_type := declared_block if is_runtime_type(checker, declared_block) else types.INVALID
 				local := Infer_Local{
 					name=statement.name, type=block_type, declared=declared_block,
@@ -1876,7 +2021,7 @@ infer_statements :: proc(
 				infer_statements(checker, statement.body, locals, local_types, pkg, file, demanded, result, result_hint)
 				continue
 			}
-			declared_local := resolve_inferred_array(checker, type_from_syntax(statement.type), statement.expr)
+			declared_local := resolve_inferred_array(checker, type_from_syntax(checker, statement.type, pkg, file), statement.expr)
 			value_type := types.INVALID
 			if !is_undefined_expr(checker, statement.expr) {
 				value_type = infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded, local_types)
@@ -2107,7 +2252,7 @@ infer_spec_locals_and_result :: proc(
 ) -> ([]types.Type, types.Type) {
 	spec := checker.specs[id]
 	function := checker.ast_module.functions[spec.template]
-	declared := type_from_syntax(function.result)
+	declared := type_from_syntax(checker, function.result, function.pkg, function.file)
 	if function.pkg == 0 && function.name == checker.main_symbol && function.result == types.INT {
 		declared = types.I32
 	}
@@ -2446,7 +2591,7 @@ infer_all :: proc(checker: ^Checker) {
 	// Demands accumulate in global_demands so the default never blocks a later
 	// cross-family demand (e.g. integer literal -> unsigned or float).
 	for global, index in checker.ast_module.globals {
-		declared := type_from_syntax(global.type)
+		declared := type_from_syntax(checker, global.type, global.pkg, global.file)
 		if is_runtime_type(checker, declared) {
 			checker.global_types[index] = declared
 			continue
@@ -2494,7 +2639,7 @@ infer_all :: proc(checker: ^Checker) {
 				continue
 			}
 			inferred := infer_expr(checker, global.expr, nil, global.pkg, global.file)
-			if is_runtime_type(checker, type_from_syntax(global.type)) {
+			if is_runtime_type(checker, type_from_syntax(checker, global.type, global.pkg, global.file)) {
 				continue
 			}
 			if is_runtime_type(checker, checker.global_demands[index]) {
@@ -2660,9 +2805,11 @@ can_implicitly_convert_type :: proc(checker: ^Checker, actual, expected: types.T
 	store := &checker.module.types
 	if types.equal(actual, expected) ||
 	   types.can_widen(actual, expected) ||
-	   types.can_coerce_c_integer(actual, expected) ||
+	   types.can_coerce_c_integer(actual, expected, checker.target) ||
+	   types.can_coerce_c_scalar(actual, expected, checker.target) ||
 	   types.can_weaken_pointer(actual, expected, store) ||
 	   types.can_weaken_slice(actual, expected, store) ||
+	   types.can_decay_slice_c_string(actual, expected, store) ||
 	   types.can_decay_array_pointer(actual, expected, store) ||
 	   types.can_sum_widen(actual, expected, store) {
 		return true
@@ -2719,6 +2866,17 @@ coerce_expr :: proc(
 			diagnostic=source.INVALID_DIAGNOSTIC,
 		})
 	}
+	if types.can_decay_slice_c_string(actual, expected, &checker.module.types) {
+		return add_hir_expr(checker, hir.Expr{
+			kind=.Slice_Ptr,
+			span=span,
+			type=expected,
+			left=expr_id,
+			target=hir.INVALID_REF,
+			right=hir.INVALID_EXPR,
+			diagnostic=source.INVALID_DIAGNOSTIC,
+		})
+	}
 	if types.can_sum_widen(actual, expected, &checker.module.types) {
 		return add_hir_expr(checker, hir.Expr{
 			kind=.Sum_Widen,
@@ -2734,8 +2892,11 @@ coerce_expr :: proc(
 		child := types.child_type(expected, &checker.module.types)
 		if types.equal(actual, child) ||
 		   types.can_widen(actual, child) ||
+		   types.can_coerce_c_integer(actual, child, checker.target) ||
+		   types.can_coerce_c_scalar(actual, child, checker.target) ||
 		   types.can_weaken_pointer(actual, child, &checker.module.types) ||
 		   types.can_weaken_slice(actual, child, &checker.module.types) ||
+		   types.can_decay_slice_c_string(actual, child, &checker.module.types) ||
 		   types.can_decay_array_pointer(actual, child, &checker.module.types) {
 			value := coerce_expr(checker, expr_id, child, span)
 			return add_hir_expr(checker, hir.Expr{
@@ -2763,7 +2924,8 @@ coerce_expr :: proc(
 			},
 		)
 	}
-	if types.can_coerce_c_integer(actual, expected) {
+	if types.can_coerce_c_integer(actual, expected, checker.target) ||
+	   types.can_coerce_c_scalar(actual, expected, checker.target) {
 		return add_hir_expr(
 			checker,
 			hir.Expr {
@@ -3311,6 +3473,31 @@ build_compound_expr :: proc(
 			return invalid_hir_expr(checker, expr.span, id, expected)
 		}
 		return enum_member_hir(checker, expected, expr.name, expr.span)
+	case .Cast:
+		target := type_from_syntax(checker, expr.type, pkg, file)
+		value := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
+		actual := checker.module.exprs[value].type
+		valid_target := types.is_concrete_scalar(target) && !types.is_bool(target)
+		valid_actual := types.is_concrete_scalar(actual) && !types.is_bool(actual)
+		if !valid_target || !valid_actual {
+			id := source.addf(
+				checker.diagnostics,
+				expr.span,
+				"scalar cast requires numeric scalar types, got %s to %s",
+				types.name(actual),
+				types.name(target),
+			)
+			return invalid_hir_expr(checker, expr.span, id, target)
+		}
+		return add_hir_expr(checker, hir.Expr{
+			kind=.Scalar_Cast,
+			span=expr.span,
+			type=target,
+			left=value,
+			target=hir.INVALID_REF,
+			right=hir.INVALID_EXPR,
+			diagnostic=source.INVALID_DIAGNOSTIC,
+		})
 	case .Address:
 		value := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
 		if !hir_is_location(checker, value) {
@@ -3343,6 +3530,7 @@ build_compound_expr :: proc(
 	case .Index:
 		container := build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
 		index := build_nested_expr(checker, expr.right, locals, global_reads, calls, types.USIZE, pkg, file)
+		index = coerce_expr(checker, index, types.USIZE, expr.span)
 		container_type := checker.module.exprs[container].type
 		item, ok := types.container(container_type, store)
 		if !ok {
@@ -3367,6 +3555,7 @@ build_compound_expr :: proc(
 		for bound, index in expr.args {
 			if bound != ast.INVALID_EXPR {
 				bounds[index] = build_nested_expr(checker, bound, locals, global_reads, calls, types.USIZE, pkg, file)
+				bounds[index] = coerce_expr(checker, bounds[index], types.USIZE, checker.ast_module.exprs[bound].span)
 			}
 		}
 		preserve_sentinel := item.has_sentinel && expr.args[1] == ast.INVALID_EXPR
@@ -3603,17 +3792,19 @@ build_compound_expr :: proc(
 		left, right: hir.Expr_Id
 		left_expr := checker.ast_module.exprs[expr.left]
 		right_expr := checker.ast_module.exprs[expr.right]
+		left_numeric_const := left_const.kind == .Value || is_float_constant_expr(checker, expr.left)
+		right_numeric_const := right_const.kind == .Value || is_float_constant_expr(checker, expr.right)
 		if right_expr.kind == .Enum_Literal && left_expr.kind != .Enum_Literal {
 			left = build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
 			right = build_nested_expr(checker, expr.right, locals, global_reads, calls, checker.module.exprs[left].type, pkg, file)
 		} else if left_expr.kind == .Enum_Literal && right_expr.kind != .Enum_Literal {
 			right = build_nested_expr(checker, expr.right, locals, global_reads, calls, types.INVALID, pkg, file)
 			left = build_nested_expr(checker, expr.left, locals, global_reads, calls, checker.module.exprs[right].type, pkg, file)
-		} else if right_const.kind == .Value && left_const.kind != .Value {
+		} else if right_numeric_const && !left_numeric_const {
 			left = build_nested_expr(checker, expr.left, locals, global_reads, calls, types.INVALID, pkg, file)
 			hint := checker.module.exprs[left].type
 			right = build_nested_expr(checker, expr.right, locals, global_reads, calls, hint, pkg, file)
-		} else if left_const.kind == .Value && right_const.kind != .Value {
+		} else if left_numeric_const && !right_numeric_const {
 			right = build_nested_expr(checker, expr.right, locals, global_reads, calls, types.INVALID, pkg, file)
 			hint := checker.module.exprs[right].type
 			left = build_nested_expr(checker, expr.left, locals, global_reads, calls, hint, pkg, file)
@@ -3841,7 +4032,7 @@ build_expr :: proc(
 			switch expr.kind {
 	case .String, .Array, .None, .Undefined, .Address, .Deref, .Index, .Slice,
 			     .Field, .Unwrap, .Orelse, .Try, .Catch, .Struct_Literal, .Keyed,
-			     .Bool, .Not, .Eq, .Ne, .Lt, .Le, .Gt, .Ge, .And, .Or, .Range,
+			     .Bool, .Cast, .Not, .Eq, .Ne, .Lt, .Le, .Gt, .Ge, .And, .Or, .Range,
 			     .Enum_Literal:
 				last = build_compound_expr(
 					checker, expr, locals, global_reads, calls, frame.expected, pkg, file,
@@ -4086,7 +4277,7 @@ build_expr :: proc(
 				stack[frame_index].arg_types = make([]types.Type, len(expr.args), checker.allocator)
 				stack[frame_index].stage = 3
 				if len(expr.args) > 0 {
-					arg_expected := call_arg_expected(function, 0)
+					arg_expected := call_arg_expected(checker, function, 0)
 					if !is_runtime_type(checker, arg_expected) {
 						arg_expected = types.INVALID
 					}
@@ -4141,7 +4332,7 @@ build_expr :: proc(
 				stack[frame_index].arg_index += 1
 				if frame.arg_index+1 < len(expr.args) {
 					next := frame.arg_index+1
-					arg_expected := call_arg_expected(checker.ast_module.functions[frame.template], next)
+					arg_expected := call_arg_expected(checker, checker.ast_module.functions[frame.template], next)
 					if !is_runtime_type(checker, arg_expected) {
 						arg_expected = types.INVALID
 					}
@@ -4156,7 +4347,7 @@ build_expr :: proc(
 					if index >= len(stack[frame_index].arg_types) {
 						break
 					}
-					declared := type_from_syntax(params[index].type)
+					declared := type_from_syntax(checker, params[index].type, checker.ast_module.functions[frame.template].pkg, checker.ast_module.functions[frame.template].file)
 					actual := stack[frame_index].arg_types[index]
 					if types.is_constraint(declared) && types.is_valid(actual) &&
 					   !types.is_valid(types.constraint_target(declared, actual, &checker.module.types)) {
@@ -4413,9 +4604,9 @@ build_block :: proc(
 			// declare the local from the yielded value (its type for an untyped `::`).
 			if statement.expr == ast.INVALID_EXPR {
 				expected := types.INVALID
-				typed := is_runtime_type(checker, type_from_syntax(statement.type))
+				typed := is_runtime_type(checker, type_from_syntax(checker, statement.type, ctx.pkg, ctx.file))
 				if typed {
-					expected = type_from_syntax(statement.type)
+					expected = type_from_syntax(checker, statement.type, ctx.pkg, ctx.file)
 				}
 				value, value_type := build_value_source(ctx, &body, statement.body, expected, statement.span, statement.label, statement.value_control_flow)
 				if _, found := find_build_local(ctx.locals^[duplicate_start:], statement.name); found {
@@ -4445,7 +4636,7 @@ build_block :: proc(
 				})
 				continue
 			}
-			declared := resolve_inferred_array(checker, type_from_syntax(statement.type), statement.expr)
+			declared := resolve_inferred_array(checker, type_from_syntax(checker, statement.type, ctx.pkg, ctx.file), statement.expr)
 			// Adopt the type inference resolved for this local when the declaration has no
 			// concrete annotation and inference carried useful numeric context: constraints,
 			// `undefined`, open numeric constants, or arithmetic expressions.
@@ -4717,7 +4908,7 @@ build_block :: proc(
 				ctx.problematic^ = true
 				continue
 			}
-			if statement.expr == ast.INVALID_EXPR {
+			if statement.expr == ast.INVALID_EXPR && !statement.value_control_flow {
 				if types.kind(ctx.result, store) == .Fallible &&
 				   types.is_void(types.fallible_success(ctx.result, store)) {
 					flush_defers(ctx, &body, 0)
@@ -4756,7 +4947,17 @@ build_block :: proc(
 				continue
 			}
 			value := hir.INVALID_EXPR
-			if types.kind(ctx.result, store) == .Fallible {
+			if statement.value_control_flow {
+				if types.kind(ctx.result, store) == .Fallible {
+					success := types.fallible_success(ctx.result, store)
+					value, _ = build_value_source(ctx, &body, statement.body, success, statement.span, symbol.INVALID, statement.value_control_flow)
+					value = coerce_expr(checker, value, success, statement.span)
+					value = fallible_aggregate(checker, statement.span, ctx.result, value, false)
+				} else {
+					value, _ = build_value_source(ctx, &body, statement.body, ctx.result, statement.span, symbol.INVALID, statement.value_control_flow)
+					value = coerce_expr(checker, value, ctx.result, statement.span)
+				}
+			} else if types.kind(ctx.result, store) == .Fallible {
 				success := types.fallible_success(ctx.result, store)
 				error_type := types.fallible_error(ctx.result, store)
 				error_path := false
@@ -5249,7 +5450,12 @@ build_block :: proc(
 					continue
 				}
 				target := &ctx.yield_targets^[target_index]
-				yielded := build_expr(checker, statement.expr, ctx.locals^[:], ctx.global_reads, ctx.calls, target.slot_type, ctx.pkg, ctx.file)
+				yielded := hir.INVALID_EXPR
+				if statement.value_control_flow {
+					yielded, _ = build_value_source(ctx, &body, statement.body, target.slot_type, statement.span, symbol.INVALID, statement.value_control_flow)
+				} else {
+					yielded = build_expr(checker, statement.expr, ctx.locals^[:], ctx.global_reads, ctx.calls, target.slot_type, ctx.pkg, ctx.file)
+				}
 				yielded = resolve_loop_slot(ctx, target, yielded, checker.module.exprs[yielded].type if yielded != hir.INVALID_EXPR else types.INVALID, statement.span)
 				if target.slot == hir.INVALID_LOCAL || yielded == hir.INVALID_EXPR {
 					id := source.add(checker.diagnostics, statement.span,
@@ -5391,10 +5597,14 @@ build_value_block :: proc(
 	delete(leading, checker.allocator)
 
 	yield_stmt := checker.ast_module.statements[body_stmts[n - 1]]
-	value = build_expr(
+	if yield_stmt.value_control_flow {
-		checker, yield_stmt.expr, ctx.locals^[:], ctx.global_reads, ctx.calls,
+		value, value_type = build_value_source(ctx, body, yield_stmt.body, expected, yield_stmt.span, symbol.INVALID, yield_stmt.value_control_flow)
-		expected, ctx.pkg, ctx.file,
+	} else {
-	)
+		value = build_expr(
+			checker, yield_stmt.expr, ctx.locals^[:], ctx.global_reads, ctx.calls,
+			expected, ctx.pkg, ctx.file,
+		)
+	}
 	value_type = checker.module.exprs[value].type
 	if is_runtime_type(checker, expected) {
 		value = coerce_expr(checker, value, expected, yield_stmt.span)
@@ -5486,6 +5696,38 @@ emit_slot_assign :: proc(checker: ^Checker, out: ^[dynamic]hir.Stmt_Id, slot: hi
 	})
 }
 
+string_peer_slice_type :: proc(checker: ^Checker, value: types.Type) -> types.Type {
+	pointer, array, ok := types.array_pointer(value, &checker.module.types)
+	if ok && !pointer.mutable && !array.mutable &&
+	   array.child == types.U8 && array.has_sentinel && array.sentinel == 0 {
+		return types.slice(&checker.module.types, types.U8, false, true, 0)
+	}
+	return types.INVALID
+}
+
+adopt_value_slot :: proc(
+	ctx: ^Build_Ctx,
+	slot: ^hir.Local_Id,
+	slot_type: ^types.Type,
+	value: hir.Expr_Id,
+	vtype: types.Type,
+	span: source.Span,
+) -> hir.Expr_Id {
+	checker := ctx.checker
+	if slot^ == hir.INVALID_LOCAL {
+		peer := string_peer_slice_type(checker, vtype)
+		if types.is_valid(peer) {
+			slot_type^ = peer
+			slot^ = new_value_slot(ctx, slot_type^)
+			return coerce_expr(checker, value, slot_type^, span)
+		}
+		slot_type^ = vtype
+		slot^ = new_value_slot(ctx, slot_type^)
+		return value
+	}
+	return coerce_expr(checker, value, slot_type^, span)
+}
+
 // build_value_if turns `if c { … yield A } else { … yield B }` into a result slot
 // each branch assigns, read after the if. Every path must yield: a mandatory `else`,
 // each branch ends in `yield`, and all branches share a type (the first establishes it
@@ -5703,12 +5945,7 @@ emit_value_branch :: proc(
 	if checker.module.exprs[value].kind == .Invalid {
 		return false
 	}
-	if slot^ == hir.INVALID_LOCAL {
+	value = adopt_value_slot(ctx, slot, slot_type, value, vtype, span)
-		slot_type^ = vtype
-		slot^ = new_value_slot(ctx, slot_type^)
-	} else {
-		value = coerce_expr(checker, value, slot_type^, span)
-	}
 	emit_slot_assign(checker, out, slot^, value, span)
 	return true
 }
@@ -6192,12 +6429,7 @@ build_value_arm :: proc(
 			return false
 		}
 		vtype := checker.module.exprs[value].type
-		if slot^ == hir.INVALID_LOCAL {
+		value = adopt_value_slot(ctx, slot, slot_type, value, vtype, span)
-			slot_type^ = vtype
-			slot^ = new_value_slot(ctx, slot_type^)
-		} else {
-			value = coerce_expr(checker, value, slot_type^, span)
-		}
 		emit_slot_assign(checker, out, slot^, value, span)
 		return true
 	}
@@ -6937,7 +7169,7 @@ build_globals :: proc(checker: ^Checker) {
 		dependencies.allocator = checker.allocator
 		calls: [dynamic]hir.Function_Id
 		calls.allocator = checker.allocator
-		declared := resolve_inferred_array(checker, type_from_syntax(global.type), global.expr)
+		declared := resolve_inferred_array(checker, type_from_syntax(checker, global.type, global.pkg, global.file), global.expr)
 		expected := types.INVALID
 		if is_runtime_type(checker, declared) {
 			expected = declared

compiler/hir/hir.odin

@@ -102,6 +102,7 @@ Expr_Kind :: enum u8 {
 	C_Coerce,
 	C_Vararg_Promote,
 	Retype,
+	Scalar_Cast,
 	Weaken_Pointer,
 	Weaken_Slice,
 	Decay_Array_Pointer,

compiler/ir/ir.odin

@@ -99,6 +99,7 @@ Opcode :: enum u8 {
 	C_Coerce,
 	C_Vararg_Promote,
 	Retype,
+	Scalar_Cast,
 	Weaken_Pointer,
 	Weaken_Slice,
 	Decay_Array_Pointer,

compiler/llvm/llvm.odin

@@ -256,7 +256,7 @@ valid_value :: proc(
 	     .Load_Global, .Function_Address, .Address_Of, .Load, .Union_Tag, .Slice, .Length, .Slice_Ptr,
 	     .Fallible_Error, .Extract, .Select, .Unwrap,
 	     .Optional_Is_Some, .Optional_Value, .Orelse,
-	     .Widen, .Sum_Widen, .C_Coerce, .C_Vararg_Promote, .Retype, .Weaken_Pointer, .Weaken_Slice, .Decay_Array_Pointer,
+	     .Widen, .Sum_Widen, .C_Coerce, .C_Vararg_Promote, .Retype, .Scalar_Cast, .Weaken_Pointer, .Weaken_Slice, .Decay_Array_Pointer,
 	     .Neg_Checked, .Add_Checked, .Sub_Checked, .Mul_Checked, .Div_Checked, .Pointer_Add, .Not, .Compare, .Call:
 		return true
 	case .Address_Global, .Alloca, .Index_Address, .Field_Address, .Orelse_Begin,
@@ -1403,16 +1403,15 @@ emit_instruction_stream :: proc(
 			emit_recovery_value(emitter, instruction_index, instruction, "unsupported sum widening operand")
 		case .C_Coerce:
 			if !valid_instruction(instructions, instruction.a) ||
-			   !types.can_coerce_c_integer(instructions[instruction.a].type, instruction.type) {
+			   !(types.can_coerce_c_integer(instructions[instruction.a].type, instruction.type, emitter.module.target) ||
-				emit_recovery_value(emitter, instruction_index, instruction, "invalid C integer coercion operand")
+			     types.can_coerce_c_scalar(instructions[instruction.a].type, instruction.type, emitter.module.target)) {
+				emit_recovery_value(emitter, instruction_index, instruction, "invalid C scalar coercion operand")
 				continue
 			}
 			from_type := instructions[instruction.a].type
 			from_bits := types.bits(from_type, emitter.module.target)
 			to_bits := types.bits(instruction.type, emitter.module.target)
 			if from_bits == to_bits {
-				// Same-width signedness change: c_uint and c_int both lower to the
-				// identical `iN`, so this is a pure reinterpret (no-op `select`).
 				type_name := llvm_type(instruction.type, &emitter.module.types)
 				fmt.sbprintf(&emitter.builder, "  %%v%d = select i1 true, %s ", instruction_index, type_name)
 				write_operand(&emitter.builder, instructions, instruction.a, from_type, &emitter.module.types)
@@ -1421,7 +1420,8 @@ emit_instruction_stream :: proc(
 				strings.write_string(&emitter.builder, "\n")
 				continue
 			}
-			operation := "sext" if types.is_signed(from_type, emitter.module.target) else "zext"
+			operation := "fpext" if types.is_float(from_type, emitter.module.target) else
+			             ("sext" if types.is_signed(from_type, emitter.module.target) else "zext")
 			fmt.sbprintf(&emitter.builder, "  %%v%d = %s %s ", instruction_index, operation, llvm_type(from_type, &emitter.module.types))
 			write_operand(&emitter.builder, instructions, instruction.a, from_type, &emitter.module.types)
 			fmt.sbprintf(&emitter.builder, " to %s\n", llvm_type(instruction.type, &emitter.module.types))
@@ -1467,6 +1467,43 @@ emit_instruction_stream :: proc(
 				&emitter.module.types,
 			)
 			fmt.sbprintf(&emitter.builder, ", %s zeroinitializer\n", type_name)
+		case .Scalar_Cast:
+			if !valid_instruction(instructions, instruction.a) ||
+			   !types.is_concrete_scalar(instructions[instruction.a].type) ||
+			   !types.is_concrete_scalar(instruction.type) ||
+			   types.is_bool(instructions[instruction.a].type) ||
+			   types.is_bool(instruction.type) {
+				emit_recovery_value(emitter, instruction_index, instruction, "invalid scalar cast operand")
+				continue
+			}
+			from_type := instructions[instruction.a].type
+			from_bits := types.bits(from_type, emitter.module.target)
+			to_bits := types.bits(instruction.type, emitter.module.target)
+			from_float := types.is_float(from_type, emitter.module.target)
+			to_float := types.is_float(instruction.type, emitter.module.target)
+			if types.equal(from_type, instruction.type) || from_bits == to_bits && from_float == to_float {
+				type_name := llvm_type(instruction.type, &emitter.module.types)
+				fmt.sbprintf(&emitter.builder, "  %%v%d = select i1 true, %s ", instruction_index, type_name)
+				write_operand(&emitter.builder, instructions, instruction.a, from_type, &emitter.module.types)
+				fmt.sbprintf(&emitter.builder, ", %s ", type_name)
+				write_operand(&emitter.builder, instructions, instruction.a, from_type, &emitter.module.types)
+				strings.write_string(&emitter.builder, "\n")
+				continue
+			}
+			operation := ""
+			switch {
+			case from_float && to_float:
+				operation = "fpext" if from_bits < to_bits else "fptrunc"
+			case !from_float && !to_float:
+				operation = "trunc" if from_bits > to_bits else ("sext" if types.is_signed(from_type, emitter.module.target) else "zext")
+			case from_float:
+				operation = "fptosi" if types.is_signed(instruction.type, emitter.module.target) else "fptoui"
+			case:
+				operation = "sitofp" if types.is_signed(from_type, emitter.module.target) else "uitofp"
+			}
+			fmt.sbprintf(&emitter.builder, "  %%v%d = %s %s ", instruction_index, operation, llvm_type(from_type, &emitter.module.types))
+			write_operand(&emitter.builder, instructions, instruction.a, from_type, &emitter.module.types)
+			fmt.sbprintf(&emitter.builder, " to %s\n", llvm_type(instruction.type, &emitter.module.types))
 		case .Weaken_Pointer:
 			if !valid_instruction(instructions, instruction.a) ||
 			   !types.can_weaken_pointer(instructions[instruction.a].type, instruction.type, &emitter.module.types) {

compiler/lower/lower.odin

@@ -665,7 +665,7 @@ lower_expr :: proc(state: ^State, expr_id: hir.Expr_Id) -> ir.Instruction_Id {
 					})
 				}
 				_ = pop(&stack)
-			case .Widen, .Sum_Widen, .C_Coerce, .C_Vararg_Promote, .Retype, .Weaken_Pointer, .Weaken_Slice, .Decay_Array_Pointer:
+			case .Widen, .Sum_Widen, .C_Coerce, .C_Vararg_Promote, .Retype, .Scalar_Cast, .Weaken_Pointer, .Weaken_Slice, .Decay_Array_Pointer:
 				stack[frame_index].stage = 1
 				append(&stack, Lower_Expr_Frame{expr=expr.left})
 			case .Negate:
@@ -726,6 +726,7 @@ lower_expr :: proc(state: ^State, expr_id: hir.Expr_Id) -> ir.Instruction_Id {
 			case .C_Coerce:            op = .C_Coerce
 			case .C_Vararg_Promote:    op = .C_Vararg_Promote
 			case .Retype:              op = .Retype
+			case .Scalar_Cast:         op = .Scalar_Cast
 			case:                      op = .Widen
 			}
 			last = append_instruction(state, ir.Instruction{

compiler/parser/parser.odin

@@ -212,9 +212,15 @@ parse_type_atom :: proc(parser: ^Parser) -> ast.Type_Syntax {
 			if _, ok := allow(parser, .Underscore); ok {
 				node.inferred_count = true
 			} else {
-				count, ok := parse_type_constant(parser)
+				if (current(parser).kind == .Integer || current(parser).kind == .Character) &&
-				if ok {
+				   (peek(parser).kind == .Right_Bracket || peek(parser).kind == .Semicolon) {
+					count, ok := parse_type_constant(parser)
 					node.count = count
+					_ = ok
+				} else {
+					expr := parse_expression(parser)
+					node.count_expr = u32(expr)
+					node.unresolved_count = true
 				}
 			}
 			if _, ok := allow(parser, .Semicolon); ok {
@@ -585,6 +591,37 @@ parse_integer_magnitude :: proc(text: string) -> (u64, bool) {
 parse_primary :: proc(parser: ^Parser, nesting: int) -> ast.Expr_Id {
 	tok := current(parser)
 	#partial switch tok.kind {
+	case .Keyword_I8, .Keyword_I16, .Keyword_I32, .Keyword_I64,
+	     .Keyword_U8, .Keyword_U16, .Keyword_U32, .Keyword_U64,
+	     .Keyword_Isize, .Keyword_Usize, .Keyword_F32, .Keyword_F64,
+	     .Keyword_C_Char, .Keyword_C_Schar, .Keyword_C_Uchar,
+	     .Keyword_C_Short, .Keyword_C_Ushort, .Keyword_C_Int, .Keyword_C_Uint,
+	     .Keyword_C_Long, .Keyword_C_Ulong, .Keyword_C_Longlong, .Keyword_C_Ulonglong,
+	     .Keyword_C_Float, .Keyword_C_Double, .Keyword_C_Longdouble:
+		start := tok
+		target := parse_type_atom(parser)
+		if _, ok := allow(parser, .Left_Paren); !ok {
+			return invalid_expr(parser, current(parser).span, "expected '(' after scalar cast type")
+		}
+		parser.delimiter_depth += 1
+		skip_newlines(parser)
+		operand := parse_expression_bp(parser, 0, nesting+1)
+		skip_newlines(parser)
+		end := current(parser)
+		if close, ok := allow(parser, .Right_Paren); ok {
+			end = close
+		} else {
+			source.add(parser.diagnostics, current(parser).span, "expected ')' after scalar cast")
+		}
+		parser.delimiter_depth -= 1
+		return add_expr(parser, ast.Expr{
+			kind=.Cast,
+			span=span_from(start.span, end.span),
+			type=target,
+			left=operand,
+			right=ast.INVALID_EXPR,
+			diagnostic=source.INVALID_DIAGNOSTIC,
+		})
 	case .Integer:
 		advance(parser)
 		value, ok := parse_integer_magnitude(token_text(parser, tok))
@@ -1126,6 +1163,21 @@ parse_return :: proc(parser: ^Parser) -> ast.Stmt_Id {
 		})
 		return id
 	}
+	if cf, is_cf := parse_value_control_flow(parser); is_cf {
+		cf_span := parser.module.statements[cf].span
+		body := make([]ast.Stmt_Id, 1, parser.module.allocator)
+		body[0] = cf
+		id := ast.stmt_id(len(parser.module.statements))
+		append(&parser.module.statements, ast.Stmt{
+			kind=.Return,
+			span=span_from(start.span, cf_span),
+			expr=ast.INVALID_EXPR,
+			body=body,
+			value_control_flow=true,
+			diagnostic=source.INVALID_DIAGNOSTIC,
+		})
+		return id
+	}
 	expr := parse_expression(parser)
 	id := ast.stmt_id(len(parser.module.statements))
 	append(&parser.module.statements, ast.Stmt{
@@ -1154,6 +1206,22 @@ parse_yield :: proc(parser: ^Parser) -> ast.Stmt_Id {
 			source.add(parser.diagnostics, current(parser).span, "expected a loop label after ':'")
 		}
 	}
+	if cf, is_cf := parse_value_control_flow(parser); is_cf {
+		cf_span := parser.module.statements[cf].span
+		body := make([]ast.Stmt_Id, 1, parser.module.allocator)
+		body[0] = cf
+		id := ast.stmt_id(len(parser.module.statements))
+		append(&parser.module.statements, ast.Stmt{
+			kind=.Yield,
+			span=span_from(start.span, cf_span),
+			label=label,
+			expr=ast.INVALID_EXPR,
+			body=body,
+			value_control_flow=true,
+			diagnostic=source.INVALID_DIAGNOSTIC,
+		})
+		return id
+	}
 	expr := parse_expression(parser)
 	id := ast.stmt_id(len(parser.module.statements))
 	append(&parser.module.statements, ast.Stmt{

compiler/types/types.odin

@@ -80,6 +80,7 @@ Node :: struct {
 	child:        Type,
 	extra:        Type,
 	count:        u64,
+	count_expr:   u32,
 	sentinel:     u64,
 	explicit_size: u64,
 	field_start:  u32,
@@ -93,6 +94,7 @@ Node :: struct {
 	many:         bool,
 	has_sentinel: bool,
 	inferred_count: bool,
+	unresolved_count: bool,
 	c_abi:        bool,
 	variadic:     bool,
 	c_layout:     bool,
@@ -1308,6 +1310,8 @@ with_array_count :: proc(store: ^Store, value: Type, count: u64) -> Type {
 	}
 	item.count = count
 	item.inferred_count = false
+	item.unresolved_count = false
+	item.count_expr = 0
 	return intern(store, item)
 }
 
@@ -1339,6 +1343,16 @@ can_weaken_slice :: proc(from, to: Type, store: ^Store) -> bool {
 	        (from_node.has_sentinel && from_node.sentinel == to_node.sentinel))
 }
 
+can_decay_slice_c_string :: proc(from, to: Type, store: ^Store) -> bool {
+	from_node, from_ok := node(store, from)
+	to_node, to_ok := node(store, to)
+	return from_ok && to_ok &&
+	       from_node.kind == .Slice && to_node.kind == .Pointer && to_node.many &&
+	       from_node.child == U8 && to_node.child == C_CHAR &&
+	       from_node.has_sentinel && from_node.sentinel == 0 && !from_node.mutable &&
+	       !to_node.mutable
+}
+
 can_decay_array_pointer :: proc(from, to: Type, store: ^Store) -> bool {
 	from_pointer, array, from_ok := array_pointer(from, store)
 	to_node, to_ok := node(store, to)
@@ -1536,10 +1550,18 @@ can_widen :: proc(from, to: Type) -> bool {
 // make C interop cumbersome. Scope: widening (sext/zext) and same-width
 // signedness changes (no-op reinterpret); narrowing is intentionally excluded so
 // lossy conversions stay an error, matching brolang's trap-on-narrow philosophy.
-can_coerce_c_integer :: proc(from, to: Type) -> bool {
+can_coerce_c_integer :: proc(from, to: Type, selected := target.DEFAULT) -> bool {
 	return from != to && is_c(from) && is_c(to) &&
 	       is_concrete_integer(from) && is_concrete_integer(to) &&
-	       bits(from) <= bits(to)
+	       bits(from, selected) <= bits(to, selected)
+}
+
+can_coerce_c_scalar :: proc(from, to: Type, selected := target.DEFAULT) -> bool {
+	return from != to && !is_c(from) && is_c(to) &&
+	       is_concrete_scalar(from) && is_concrete_scalar(to) &&
+	       category(from, selected) == category(to, selected) &&
+	       category(from, selected) != .None &&
+	       bits(from, selected) <= bits(to, selected)
 }
 
 widest :: proc(a, b: Type) -> Type {

compiler_tests.odin

@@ -2033,6 +2033,35 @@ sentinel_pointer_views_compile_and_run :: proc(t: ^testing.T) {
 	testing.expect_value(t, state.exit_code, 303)
 }
 
+@(test)
+milestone_24_regressions_compile_and_run :: proc(t: ^testing.T) {
+	output := "/tmp/brolang-test-milestone-24"
+	defer _ = os.remove(output)
+	status := compiler_core.compile_package("examples/programs/milestone_24", output)
+	testing.expect_value(t, status, 0)
+	state := run_executable(output)
+	testing.expect_value(t, state.exit_code, 0)
+}
+
+@(test)
+milestone_24_rejects_invalid_forms :: proc(t: ^testing.T) {
+	Case :: struct {
+		directory: string,
+		output:    string,
+	}
+	cases := [?]Case{
+		{"examples/programs/index_signed_error", "/tmp/brolang-test-index-signed-error"},
+		{"examples/programs/index_int_constraint_error", "/tmp/brolang-test-index-int-constraint-error"},
+		{"examples/programs/scalar_cast_error", "/tmp/brolang-test-scalar-cast-error"},
+		{"examples/programs/array_const_size_error", "/tmp/brolang-test-array-const-size-error"},
+	}
+	for test_case in cases {
+		status := compiler_core.compile_package(test_case.directory, test_case.output)
+		testing.expect_value(t, status, 1)
+		_ = os.remove(test_case.output)
+	}
+}
+
 @(test)
 control_flow_compiles_and_runs :: proc(t: ^testing.T) {
 	output := "/tmp/brolang-test-control-flow"

examples/programs/array_const_size_error/main.bro

@@ -0,0 +1,5 @@
+main func() i32 {
+	n usize = 4
+	items [n]mut i32 = undefined
+	return 0
+}

examples/programs/index_int_constraint_error/main.bro

@@ -0,0 +1,6 @@
+main func() i32 {
+	items [3]mut i32 = undefined
+	i int = 1
+	items[i] = 42
+	return 0
+}

examples/programs/index_signed_error/main.bro

@@ -0,0 +1,6 @@
+main func() i32 {
+	items [3]mut i32 = undefined
+	i i32 = 1
+	items[i] = 42
+	return 0
+}

examples/programs/milestone_24/main.bro

@@ -0,0 +1,93 @@
+CAP :: 4
+LEN :: CAP + 1
+
+Kind :: enum {
+	circle
+	square
+	triangle
+}
+
+take_c_int func(value c_int) c_int {
+	return value
+}
+
+take_c_uchar func(value c_uchar) c_uchar {
+	return value
+}
+
+take_c_float func(value c_float) c_float {
+	return value
+}
+
+take_c_double func(value c_double) c_double {
+	return value
+}
+
+take_c_string func(value ?*c_char) i32 {
+	_ = value
+	return 0
+}
+
+next_kind func(k Kind) Kind {
+	return match k {
+		.circle:   .square
+		.square:   .triangle
+		.triangle: .circle
+	}
+}
+
+score_for func(k Kind) i32 {
+	score :: {
+		yield match k {
+			.circle:   1
+			.square:   2
+			.triangle: 3
+		}
+	}
+	return score
+}
+
+main func() i32 {
+	items [LEN]mut i32 = undefined
+	items[0] = 10
+	items[1] = 20
+
+	idx u8 = 2
+	items[idx] = items[0] + items[1]
+	if (items[2] != 30) return 1
+
+	native_i i32 = 12
+	if (take_c_int(native_i) != 12) return 2
+
+	native_u u8 = 7
+	if (take_c_uchar(native_u) != 7) return 3
+
+	native_f f32 = 3.25
+	cf :: take_c_float(native_f)
+	if (cf < 3.0 or cf > 4.0) return 4
+	if (cf == 0.0) return 5
+
+	native_d f64 = 5.0
+	cd :: take_c_double(native_d)
+	if (cd != 5.0) return 6
+
+	as_i32 :: i32(cf)
+	if (as_i32 != 3) return 7
+	as_float :: f32(native_i)
+	if (as_float < 11.5 or as_float > 12.5) return 8
+	as_c_float :: c_float(as_i32)
+	if (as_c_float != 3.0) return 9
+
+	kind :: next_kind(.circle)
+	if (kind != .square) return 10
+	if (score_for(kind) != 2) return 11
+
+	label :: match kind {
+		.circle:   "circle"
+		.square:   "square"
+		.triangle: "triangle"
+	}
+	if (take_c_string(label) != 0) return 12
+
+	return 0
+}

examples/programs/scalar_cast_error/main.bro

@@ -0,0 +1,5 @@
+main func() i32 {
+	flag bool = true
+	value :: i32(flag)
+	return value
+}