undefined assignment

TODO.md

@@ -166,6 +166,23 @@
     no hand-writable spelling and are emitted as `# unsupported in bindings:` comments
     (functions that reference an un-spellable union therefore keep a dangling reference)
 
+12. `undefined` as inspired by zig (implemented):
+  - allow mutable local declarations with `undefined`
+    - undefined values are assigned a poison value (0xaa...)
+    - allows for something like:
+      ```
+      a int = undefined
+      if (condition) {
+          a = 42
+      } else {
+          a = -2
+      }
+      ```
+  - disallow: `b :: undefined` since assigning undefined to something that can't change defeats the purpose
+  - disallow assigning `undefined` after declaration; use optionals and `none` for values that intentionally move back to an empty state
+
+13. broaden type inference from surrounding context
+
 ## A word on multi-unwrap
 
 Unwrap multiple optionals with `and`. This **short-circuits**: if the first optional is none, subsequent expressions are not evaluated.

compiler/ast/ast.odin

@@ -71,6 +71,7 @@ Expr_Kind :: enum u8 {
 	Bool,
 	Array,
 	None,
+	Undefined,
 	Name,
 	Enum_Literal,
 	Address,

compiler/checker/checker.odin

@@ -33,8 +33,11 @@ Spec :: struct {
 }
 
 Infer_Local :: struct {
-	name: symbol.Id,
+	name:      symbol.Id,
-	type: types.Type,
+	type:      types.Type,
+	declared:  types.Type,
+	statement: ast.Stmt_Id,
+	mutable:   bool,
 }
 
 Build_Local :: struct {
@@ -53,6 +56,7 @@ Build_Ctx :: struct {
 	pkg:          ast.Package_Id,
 	file:         ast.File_Id,
 	result:       types.Type,
+	local_types:  []types.Type,
 	locals:       ^[dynamic]Build_Local,
 	hir_locals:   ^[dynamic]hir.Local,
 	global_reads: ^[dynamic]hir.Global_Id,
@@ -262,6 +266,13 @@ is_runtime_type :: proc(checker: ^Checker, value: types.Type) -> bool {
 	return types.is_runtime_value(value, &checker.module.types)
 }
 
+is_undefined_expr :: proc(checker: ^Checker, expr_id: ast.Expr_Id) -> bool {
+	if expr_id == ast.INVALID_EXPR || int(expr_id) >= len(checker.ast_module.exprs) {
+		return false
+	}
+	return checker.ast_module.exprs[expr_id].kind == .Undefined
+}
+
 string_literal_type :: proc(checker: ^Checker, string_id: u64) -> types.Type {
 	length: u64
 	if string_id < u64(len(checker.ast_module.strings)) {
@@ -636,7 +647,7 @@ mark_expr_imports_used :: proc(checker: ^Checker, expr_id: ast.Expr_Id, file: as
 			append(&stack, expr.left)
 		case .Add, .Sub, .Mul, .Div, .Index, .Orelse, .Eq, .Ne, .Lt, .Le, .Gt, .Ge, .And, .Or, .Range:
 			append(&stack, expr.left, expr.right)
-		case .Invalid, .Integer, .Float, .String, .Bool, .None, .Name, .Enum_Literal:
+		case .Invalid, .Integer, .Float, .String, .Bool, .None, .Undefined, .Name, .Enum_Literal:
 		}
 	}
 }
@@ -956,14 +967,21 @@ validate_type_nodes :: proc(checker: ^Checker) {
 }
 
 find_infer_local :: proc(locals: []Infer_Local, name: symbol.Id) -> types.Type {
-	for index := len(locals) - 1; index >= 0; index -= 1 {
+	if index, ok := find_infer_local_index(locals, name); ok {
-		if locals[index].name == name {
+		return locals[index].type
-			return locals[index].type
-		}
 	}
 	return types.INVALID
 }
 
+find_infer_local_index :: proc(locals: []Infer_Local, name: symbol.Id) -> (int, bool) {
+	for index := len(locals) - 1; index >= 0; index -= 1 {
+		if locals[index].name == name {
+			return index, true
+		}
+	}
+	return -1, false
+}
+
 find_spec :: proc(checker: ^Checker, template: ast.Function_Id, actual_args: []types.Type) -> Spec_Id {
 	function := checker.ast_module.functions[template]
 	for spec, index in checker.specs {
@@ -1123,6 +1141,8 @@ infer_compound_expr :: proc(
 		return types.array(store, element, u64(len(expr.args)), false)
 	case .None:
 		return types.INVALID
+	case .Undefined:
+		return types.INVALID
 	case .Enum_Literal:
 		return types.INVALID
 	case .Address:
@@ -1247,7 +1267,7 @@ infer_expr :: proc(
 			case .Float:
 				last = types.F64
 				_ = pop(&stack)
-			case .String, .Array, .None, .Address, .Deref, .Index, .Slice,
+			case .String, .Array, .None, .Undefined, .Address, .Deref, .Index, .Slice,
 			     .Field, .Unwrap, .Orelse, .Struct_Literal, .Keyed, .Enum_Literal,
 			     .Bool, .Not, .Eq, .Ne, .Lt, .Le, .Gt, .Ge, .And, .Or, .Range:
 				last = infer_compound_expr(checker, expr, locals, pkg, file, demanded)
@@ -1505,34 +1525,127 @@ flatten_conditional_unwrap_operands :: proc(
 	append(operands, expr_id)
 }
 
+record_infer_local_type :: proc(local: Infer_Local, local_types: []types.Type) {
+	if local.statement != ast.INVALID_STMT && int(local.statement) < len(local_types) {
+		local_types[local.statement] = local.type
+	}
+}
+
+merge_infer_local_type :: proc(
+	checker: ^Checker,
+	local: ^Infer_Local,
+	inferred: types.Type,
+	local_types: []types.Type,
+) -> bool {
+	if !is_runtime_type(checker, inferred) {
+		return false
+	}
+	if types.is_constraint(local.declared) {
+		if !types.is_concrete_integer(inferred) {
+			return false
+		}
+		if !is_runtime_type(checker, local.type) {
+			local.type = inferred
+			record_infer_local_type(local^, local_types)
+			return true
+		}
+		if types.equal(local.type, inferred) {
+			return false
+		}
+		merged := types.widest(local.type, inferred)
+		if types.is_concrete_integer(merged) {
+			local.type = merged
+			record_infer_local_type(local^, local_types)
+			return true
+		}
+		return false
+	}
+	if is_runtime_type(checker, local.declared) {
+		local.type = local.declared
+		record_infer_local_type(local^, local_types)
+		return false
+	}
+	if !is_runtime_type(checker, local.type) {
+		local.type = inferred
+		record_infer_local_type(local^, local_types)
+		return true
+	}
+	if types.equal(local.type, inferred) {
+		return false
+	}
+	merged := types.widest(local.type, inferred)
+	if types.is_concrete_scalar(merged) {
+		local.type = merged
+		record_infer_local_type(local^, local_types)
+		return true
+	}
+	return false
+}
+
 infer_statements :: proc(
 	checker: ^Checker,
 	statements: []ast.Stmt_Id,
 	locals: ^[dynamic]Infer_Local,
+	local_types: []types.Type,
 	pkg: ast.Package_Id,
 	file: ast.File_Id,
 	demanded: ^[dynamic]Spec_Id,
 	result: ^types.Type,
+	result_hint := types.INVALID,
 ) {
 	scope_start := len(locals^)
 	for statement_id in statements {
 		statement := checker.ast_module.statements[statement_id]
 		#partial switch statement.kind {
 		case .Declaration:
-			value_type := infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded)
+			declared_local := resolve_inferred_array(checker, type_from_syntax(statement.type), statement.expr)
-			declared_local := type_from_syntax(statement.type)
+			value_type := types.INVALID
+			if !is_undefined_expr(checker, statement.expr) {
+				value_type = infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded)
+			}
 			if is_runtime_type(checker, declared_local) {
 				value_type = declared_local
 			}
-			append(locals, Infer_Local{name = statement.name, type = value_type})
+			local := Infer_Local{
-		case .Assignment, .Expression:
+				name=statement.name,
+				type=value_type,
+				declared=declared_local,
+				statement=statement_id,
+				mutable=!statement.immutable,
+			}
+			append(locals, local)
+			record_infer_local_type(local, local_types)
+		case .Assignment:
+			value_type := infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded)
 			if statement.target != ast.INVALID_EXPR {
 				_ = infer_expr(checker, statement.target, locals^[:], pkg, file, demanded)
+				target_expr := checker.ast_module.exprs[statement.target]
+				if target_expr.kind == .Name && !symbol.is_valid(target_expr.qualifier) {
+					if local_index, ok := find_infer_local_index(locals^[:], target_expr.name); ok &&
+					   locals^[local_index].mutable {
+						_ = merge_infer_local_type(checker, &locals^[local_index], value_type, local_types)
+					}
+				}
+			} else if statement.name != checker.sink_symbol {
+				if local_index, ok := find_infer_local_index(locals^[:], statement.name); ok &&
+				   locals^[local_index].mutable {
+					_ = merge_infer_local_type(checker, &locals^[local_index], value_type, local_types)
+				}
 			}
+		case .Expression:
 			_ = infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded)
 		case .Return:
 			if statement.expr != ast.INVALID_EXPR {
 				returned := infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded)
+				if is_runtime_type(checker, result_hint) {
+					expr := checker.ast_module.exprs[statement.expr]
+					if expr.kind == .Name && !symbol.is_valid(expr.qualifier) {
+						if local_index, ok := find_infer_local_index(locals^[:], expr.name); ok {
+							_ = merge_infer_local_type(checker, &locals^[local_index], result_hint, local_types)
+							returned = result_hint
+						}
+					}
+				}
 				if !types.is_valid(result^) {
 					result^ = returned
 				} else if !types.equal(result^, returned) {
@@ -1558,27 +1671,27 @@ infer_statements :: proc(
 					   types.is_optional(operand_types[index], &checker.module.types) {
 						capture_type = types.child_type(operand_types[index], &checker.module.types)
 					}
-					append(locals, Infer_Local{name=capture, type=capture_type})
+					append(locals, Infer_Local{name=capture, type=capture_type, declared=capture_type, statement=ast.INVALID_STMT})
 				}
 				if statement.guard != ast.INVALID_EXPR {
 					_ = infer_expr(checker, statement.guard, locals^[:], pkg, file, demanded)
 				}
-				infer_statements(checker, statement.body, locals, pkg, file, demanded, result)
+				infer_statements(checker, statement.body, locals, local_types, pkg, file, demanded, result, result_hint)
 				resize(locals, capture_start)
-				infer_statements(checker, statement.else_body, locals, pkg, file, demanded, result)
+				infer_statements(checker, statement.else_body, locals, local_types, pkg, file, demanded, result, result_hint)
 				delete(operand_types, checker.allocator)
 				delete(operands)
 			} else {
 				_ = infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded)
-				infer_statements(checker, statement.body, locals, pkg, file, demanded, result)
+				infer_statements(checker, statement.body, locals, local_types, pkg, file, demanded, result, result_hint)
-				infer_statements(checker, statement.else_body, locals, pkg, file, demanded, result)
+				infer_statements(checker, statement.else_body, locals, local_types, pkg, file, demanded, result, result_hint)
 			}
 		case .While:
 			_ = infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded)
-			infer_statements(checker, statement.body, locals, pkg, file, demanded, result)
+			infer_statements(checker, statement.body, locals, local_types, pkg, file, demanded, result, result_hint)
 			if statement.update != ast.INVALID_STMT {
 				update := [1]ast.Stmt_Id{statement.update}
-				infer_statements(checker, update[:], locals, pkg, file, demanded, result)
+				infer_statements(checker, update[:], locals, local_types, pkg, file, demanded, result, result_hint)
 			}
 		case .For:
 			iterable_type := infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded)
@@ -1596,43 +1709,55 @@ infer_statements :: proc(
 				}
 			}
 			if symbol.is_valid(statement.name) {
-				append(locals, Infer_Local{name=statement.name, type=capture_type})
+				append(locals, Infer_Local{name=statement.name, type=capture_type, declared=capture_type, statement=ast.INVALID_STMT})
 			}
 			if symbol.is_valid(statement.index_name) {
-				append(locals, Infer_Local{name=statement.index_name, type=types.USIZE})
+				append(locals, Infer_Local{name=statement.index_name, type=types.USIZE, declared=types.USIZE, statement=ast.INVALID_STMT})
 			}
-			infer_statements(checker, statement.body, locals, pkg, file, demanded, result)
+			infer_statements(checker, statement.body, locals, local_types, pkg, file, demanded, result, result_hint)
 			resize(locals, capture_start)
 		}
 	}
 	resize(locals, scope_start)
 }
 
-infer_spec_result :: proc(checker: ^Checker, id: Spec_Id, demanded: ^[dynamic]Spec_Id = nil) -> types.Type {
+infer_spec_locals_and_result :: proc(
+	checker: ^Checker,
+	id: Spec_Id,
+	demanded: ^[dynamic]Spec_Id = nil,
+) -> ([]types.Type, types.Type) {
 	spec := checker.specs[id]
 	function := checker.ast_module.functions[spec.template]
 	declared := type_from_syntax(function.result)
 	if function.pkg == 0 && function.name == checker.main_symbol && function.result == types.INT {
 		declared = types.I32
 	}
+	result_hint := declared if is_runtime_type(checker, declared) else types.INVALID
 
 	locals: [dynamic]Infer_Local
 	locals.allocator = checker.allocator
 	defer delete(locals)
+	local_types := make([]types.Type, len(checker.ast_module.statements), checker.allocator)
 	for param, index in function.params {
 		param_type := types.INVALID
 		if index < len(spec.args) {
 			param_type = spec.args[index]
 		}
-		append(&locals, Infer_Local{name = param.name, type = param_type})
+		append(&locals, Infer_Local{name=param.name, type=param_type, declared=param_type, statement=ast.INVALID_STMT})
 	}
 
 	result := types.INVALID
-	infer_statements(checker, function.body, &locals, function.pkg, function.file, demanded, &result)
+	infer_statements(checker, function.body, &locals, local_types, function.pkg, function.file, demanded, &result, result_hint)
 	if types.is_constraint(declared) {
-		return result
+		return local_types, result
 	}
-	return declared
+	return local_types, declared
+}
+
+infer_spec_result :: proc(checker: ^Checker, id: Spec_Id, demanded: ^[dynamic]Spec_Id = nil) -> types.Type {
+	local_types, result := infer_spec_locals_and_result(checker, id, demanded)
+	delete(local_types, checker.allocator)
+	return result
 }
 
 merge_inferred_type :: proc(store: ^types.Store, current: ^types.Type, inferred: types.Type) -> bool {
@@ -2296,7 +2421,7 @@ build_compound_expr :: proc(
 			infer_locals := make([]Infer_Local, len(locals), checker.allocator)
 			defer delete(infer_locals, checker.allocator)
 			for local, index in locals {
-				infer_locals[index] = Infer_Local{name=local.name, type=local.type}
+				infer_locals[index] = Infer_Local{name=local.name, type=local.type, declared=local.type, statement=ast.INVALID_STMT}
 			}
 			for arg in expr.args {
 				actual := infer_nested_expr(checker, arg, infer_locals, pkg, file, nil)
@@ -2332,6 +2457,13 @@ build_compound_expr :: proc(
 			kind=.None, span=expr.span, type=expected, target=hir.INVALID_REF,
 			left=hir.INVALID_EXPR, right=hir.INVALID_EXPR, diagnostic=source.INVALID_DIAGNOSTIC,
 		})
+	case .Undefined:
+		id := source.add(
+			checker.diagnostics,
+			expr.span,
+			"'undefined' is only valid as a mutable local declaration initializer",
+		)
+		return invalid_hir_expr(checker, expr.span, id, expected)
 	case .Enum_Literal:
 		if !types.is_enum(expected, store) {
 			id := source.addf(
@@ -2762,7 +2894,7 @@ build_expr :: proc(
 				continue
 			}
 			switch expr.kind {
-			case .String, .Array, .None, .Address, .Deref, .Index, .Slice,
+			case .String, .Array, .None, .Undefined, .Address, .Deref, .Index, .Slice,
 			     .Field, .Unwrap, .Orelse, .Struct_Literal, .Keyed,
 			     .Bool, .Not, .Eq, .Ne, .Lt, .Le, .Gt, .Ge, .And, .Or, .Range,
 			     .Enum_Literal:
@@ -3290,22 +3422,62 @@ build_block :: proc(
 		switch statement.kind {
 		case .Declaration:
 			declared := resolve_inferred_array(checker, type_from_syntax(statement.type), statement.expr)
-			expected := types.INVALID
+			if statement_id != ast.INVALID_STMT && int(statement_id) < len(ctx.local_types) &&
-			if is_runtime_type(checker, declared) {
+			   is_runtime_type(checker, ctx.local_types[statement_id]) &&
-				expected = declared
+			   (types.is_constraint(declared) || is_undefined_expr(checker, statement.expr)) {
+				declared = ctx.local_types[statement_id]
 			}
-			value := build_expr(
+			expected := types.INVALID
-				checker, statement.expr, ctx.locals^[:], ctx.global_reads, ctx.calls,
+			value := hir.INVALID_EXPR
-				expected, ctx.pkg, ctx.file,
+			value_type := types.INVALID
-			)
+			if is_undefined_expr(checker, statement.expr) {
-			value_type := checker.module.exprs[value].type
+				if statement.immutable {
-			if is_runtime_type(checker, declared) {
+					id := source.add(
-				value = coerce_expr(checker, value, declared, statement.span)
+						checker.diagnostics,
+						statement.span,
+						"'undefined' requires a mutable local declaration",
+					)
+					append(&body, hir.stmt_id(len(checker.module.statements)))
+					append(&checker.module.statements, hir.Stmt{
+						kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
+						local = hir.INVALID_LOCAL, diagnostic = id,
+					})
+					ctx.problematic^ = true
+					continue
+				}
+				if !is_runtime_type(checker, declared) {
+					id := source.addf(
+						checker.diagnostics,
+						statement.span,
+						"could not infer a concrete type for local '%s'",
+						symbol_text(checker, statement.name),
+					)
+					append(&body, hir.stmt_id(len(checker.module.statements)))
+					append(&checker.module.statements, hir.Stmt{
+						kind = .Trap, span = statement.span, expr = hir.INVALID_EXPR,
+						local = hir.INVALID_LOCAL, diagnostic = id,
+					})
+					ctx.problematic^ = true
+					continue
+				}
+				value_type = declared
+			} else {
+				if is_runtime_type(checker, declared) {
+					expected = declared
+				}
+				value = build_expr(
+					checker, statement.expr, ctx.locals^[:], ctx.global_reads, ctx.calls,
+					expected, ctx.pkg, ctx.file,
+				)
 				value_type = checker.module.exprs[value].type
-			} else if types.is_void(declared) {
+				if is_runtime_type(checker, declared) {
-				id := source.add(checker.diagnostics, statement.span, "locals cannot have type void")
+					value = coerce_expr(checker, value, declared, statement.span)
-				value = invalid_hir_expr(checker, statement.span, id)
+					value_type = checker.module.exprs[value].type
-				value_type = types.INVALID
+				} else if types.is_void(declared) {
+					id := source.add(checker.diagnostics, statement.span, "locals cannot have type void")
+					value = invalid_hir_expr(checker, statement.span, id)
+					value_type = types.INVALID
+				}
 			}
 			if _, found := find_build_local(ctx.locals^[duplicate_start:], statement.name); found {
 				id := source.addf(
@@ -3332,7 +3504,9 @@ build_block :: proc(
 				kind = .Declaration, span = statement.span, local = local_id, expr = value,
 				diagnostic = source.INVALID_DIAGNOSTIC,
 			})
-			ctx.problematic^ = ctx.problematic^ || checker.module.exprs[value].kind == .Invalid
+			if value != hir.INVALID_EXPR {
+				ctx.problematic^ = ctx.problematic^ || checker.module.exprs[value].kind == .Invalid
+			}
 		case .Assignment:
 			if statement.target != ast.INVALID_EXPR {
 				target_expr := build_expr(
@@ -3888,6 +4062,13 @@ build_function :: proc(checker: ^Checker, id: Spec_Id) {
 	global_reads.allocator = checker.allocator
 	calls: [dynamic]hir.Function_Id
 	calls.allocator = checker.allocator
+	demanded: [dynamic]Spec_Id
+	demanded.allocator = checker.allocator
+	local_types, _ := infer_spec_locals_and_result(checker, id, &demanded)
+	defer {
+		delete(local_types, checker.allocator)
+		delete(demanded)
+	}
 
 	for param, index in function.params {
 		local_id := hir.local_id(len(hir_locals))
@@ -3946,6 +4127,7 @@ build_function :: proc(checker: ^Checker, id: Spec_Id) {
 		pkg = function.pkg,
 		file = function.file,
 		result = spec.result,
+		local_types = local_types,
 		locals = &locals,
 		hir_locals = &hir_locals,
 		global_reads = &global_reads,

compiler/ir/ir.odin

@@ -81,6 +81,7 @@ Opcode :: enum u8 {
 	Field_Address,
 	Load,
 	Store,
+	Fill,
 	Slice,
 	Length,
 	Slice_Ptr,

compiler/lexer/lexer.odin

@@ -25,6 +25,7 @@ keyword_kind :: proc(text: string) -> token.Kind {
 	case "return":        return .Keyword_Return
 	case "mut":           return .Keyword_Mut
 	case "none":          return .Keyword_None
+	case "undefined":     return .Keyword_Undefined
 	case "orelse":        return .Keyword_Orelse
 	case "and":           return .Keyword_And
 	case "or":            return .Keyword_Or

compiler/llvm/llvm.odin

@@ -260,7 +260,7 @@ valid_value :: proc(
 	     .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,
-	     .Store, .Trap, .Label, .Br, .Cond_Br, .Return, .Return_Void:
+	     .Store, .Fill, .Trap, .Label, .Br, .Cond_Br, .Return, .Return_Void:
 		return false
 	}
 	return false
@@ -964,6 +964,17 @@ emit_instruction_stream :: proc(
 			fmt.sbprintf(&emitter.builder, "  store %s ", llvm_type(instruction.type, &emitter.module.types))
 			write_operand(&emitter.builder, instructions, instruction.b, instruction.type, &emitter.module.types)
 			fmt.sbprintf(&emitter.builder, ", ptr %%v%d\n", instruction.a)
+		case .Fill:
+			if !valid_address(instructions, instruction.a, instruction.type, &emitter.module.types) {
+				emit_recovery_value(emitter, instruction_index, instruction, "invalid fill slot")
+				continue
+			}
+			fmt.sbprintf(
+				&emitter.builder,
+				"  call void @llvm.memset.p0.i64(ptr %%v%d, i8 -86, i64 %d, i1 false)\n",
+				instruction.a,
+				types.size(instruction.type, &emitter.module.types, emitter.module.target),
+			)
 		case .Slice:
 			if !valid_instruction(instructions, instruction.a) {
 				emit_recovery_value(emitter, instruction_index, instruction, "invalid slice container")
@@ -1990,7 +2001,7 @@ emit_messages :: proc(emitter: ^Emitter) {
 }
 
 emit_declarations :: proc(emitter: ^Emitter) {
-	strings.write_string(&emitter.builder, "declare i64 @write(i32, ptr, i64)\ndeclare void @llvm.trap()\ndeclare void @llvm.memcpy.p0.p0.i64(ptr, ptr, i64, i1 immarg)\n")
+	strings.write_string(&emitter.builder, "declare i64 @write(i32, ptr, i64)\ndeclare void @llvm.trap()\ndeclare void @llvm.memcpy.p0.p0.i64(ptr, ptr, i64, i1 immarg)\ndeclare void @llvm.memset.p0.i64(ptr, i8, i64, i1 immarg)\n")
 	widths := [?]int{8, 16, 32, 64}
 	overflow_intrinsics := [?]string{"sadd", "uadd", "ssub", "usub", "smul", "umul"}
 	for bits in widths {

compiler/lower/lower.odin

@@ -613,7 +613,6 @@ lower_statements :: proc(state: ^State, statements: []hir.Stmt_Id) {
 		statement := hir_module.statements[statement_id]
 		switch statement.kind {
 		case .Declaration:
-			value := lower_expr(state, statement.expr)
 			if statement.local == hir.INVALID_LOCAL || int(statement.local) >= len(state.func_locals) {
 				append_instruction(state, ir.Instruction{
 					op=.Trap, span=statement.span, type=types.VOID,
@@ -632,6 +631,19 @@ lower_statements :: proc(state: ^State, statements: []hir.Stmt_Id) {
 				diagnostic=source.INVALID_DIAGNOSTIC,
 			})
 			state.local_slots[statement.local] = slot
+			if statement.expr == hir.INVALID_EXPR {
+				append_instruction(state, ir.Instruction{
+					op=.Fill,
+					span=statement.span,
+					type=local.type,
+					target=ir.INVALID_REF,
+					a=slot,
+					b=ir.INVALID_INSTRUCTION,
+					diagnostic=source.INVALID_DIAGNOSTIC,
+				})
+				continue
+			}
+			value := lower_expr(state, statement.expr)
 			append_instruction(state, ir.Instruction{
 				op=.Store,
 				span=statement.span,

compiler/parser/parser.odin

@@ -576,6 +576,15 @@ parse_primary :: proc(parser: ^Parser, nesting: int) -> ast.Expr_Id {
 			right=ast.INVALID_EXPR,
 			diagnostic=source.INVALID_DIAGNOSTIC,
 		})
+	case .Keyword_Undefined:
+		advance(parser)
+		return add_expr(parser, ast.Expr{
+			kind=.Undefined,
+			span=tok.span,
+			left=ast.INVALID_EXPR,
+			right=ast.INVALID_EXPR,
+			diagnostic=source.INVALID_DIAGNOSTIC,
+		})
 	case .Keyword_True, .Keyword_False:
 		advance(parser)
 		return add_expr(parser, ast.Expr{

compiler/token/token.odin

@@ -59,6 +59,7 @@ Kind :: enum u8 {
 	Keyword_Return,
 	Keyword_Mut,
 	Keyword_None,
+	Keyword_Undefined,
 	Keyword_Orelse,
 	Keyword_And,
 	Keyword_Or,

compiler_tests.odin

@@ -375,6 +375,32 @@ main :: func() void {}
 	testing.expect(t, strings.contains(diagnostics.items[0].message, "followed by a newline"))
 }
 
+@(test)
+parser_accepts_undefined_expression :: proc(t: ^testing.T) {
+	text := `main :: func() void {
+	value i32 = undefined
+}
+`
+	source_file := source.Source{path="test.bro", text=text}
+	diagnostics := source.init_diagnostics(&source_file)
+	defer source.destroy_diagnostics(&diagnostics)
+	symbols := symbol.init_table()
+	defer symbol.destroy_table(&symbols)
+	stream := lexer.lex(&source_file, &diagnostics, &symbols)
+	defer delete(stream.items)
+	module := parser.parse(&stream, &source_file, &diagnostics)
+	defer ast.destroy_module(&module)
+
+	found_keyword := false
+	for tok in stream.items {
+		found_keyword = found_keyword || tok.kind == .Keyword_Undefined
+	}
+	statement := module.statements[module.functions[0].body[0]]
+	testing.expect_value(t, len(diagnostics.items), 0)
+	testing.expect(t, found_keyword)
+	testing.expect_value(t, module.exprs[statement.expr].kind, ast.Expr_Kind.Undefined)
+}
+
 @(test)
 pratt_parser_preserves_left_associative_addition_shape :: proc(t: ^testing.T) {
 	source_file := source.Source{path="test.bro", text="value :: 1 + 2 + 3\nmain :: func() void {}\n"}
@@ -5013,6 +5039,187 @@ compound_assignment_preserves_operation_and_rhs :: proc(t: ^testing.T) {
 	testing.expect_value(t, module.exprs[statement.expr].integer, u64(5))
 }
 
+@(test)
+undefined_inferred_local_lowers_to_fill :: proc(t: ^testing.T) {
+	text := `choose :: func(flag bool) i32 {
+	value int = undefined
+	if flag {
+		value = 42
+	} else {
+		value = -2
+	}
+	return value
+}
+main :: func() i32 {
+	return choose(true)
+}
+`
+	source_file := source.Source{path="test.bro", text=text}
+	diagnostics := source.init_diagnostics(&source_file)
+	defer source.destroy_diagnostics(&diagnostics)
+	symbols := symbol.init_table()
+	defer symbol.destroy_table(&symbols)
+	stream := lexer.lex(&source_file, &diagnostics, &symbols)
+	defer delete(stream.items)
+	ast_module := parser.parse(&stream, &source_file, &diagnostics)
+	defer ast.destroy_module(&ast_module)
+	hir_module := checker.check(&ast_module, &diagnostics, &symbols)
+	defer hir.destroy_module(&hir_module)
+	ir_module := lower.lower(&hir_module)
+	defer ir.destroy_module(&ir_module)
+	llvm_text := llvm.emit(&ir_module, &diagnostics, &symbols)
+	defer delete(llvm_text)
+
+	choose_symbol := symbol.intern(&symbols, "choose")
+	value_symbol := symbol.intern(&symbols, "value")
+	found_value_i32 := false
+	fill_count := 0
+	for function, function_index in hir_module.functions {
+		if function.name != choose_symbol {
+			continue
+		}
+		for local in function.locals {
+			found_value_i32 = found_value_i32 || local.name == value_symbol && local.type == types.I32
+		}
+		for instruction in ir_module.functions[function_index].instructions {
+			fill_count += 1 if instruction.op == .Fill else 0
+		}
+	}
+
+	testing.expect_value(t, len(diagnostics.items), 0)
+	testing.expect(t, found_value_i32)
+	testing.expect_value(t, fill_count, 1)
+	testing.expect(t, strings.contains(llvm_text, "declare void @llvm.memset.p0.i64"))
+	testing.expect(t, strings.contains(llvm_text, "call void @llvm.memset.p0.i64"))
+	testing.expect(t, strings.contains(llvm_text, "i8 -86"))
+}
+
+@(test)
+local_int_inference_widens_from_assignments :: proc(t: ^testing.T) {
+	text := `wide :: func() int {
+	value int = 1
+	value = 1000
+	return value
+}
+main :: func() void {
+	_ = wide()
+}
+`
+	source_file := source.Source{path="test.bro", text=text}
+	diagnostics := source.init_diagnostics(&source_file)
+	defer source.destroy_diagnostics(&diagnostics)
+	symbols := symbol.init_table()
+	defer symbol.destroy_table(&symbols)
+	stream := lexer.lex(&source_file, &diagnostics, &symbols)
+	defer delete(stream.items)
+	ast_module := parser.parse(&stream, &source_file, &diagnostics)
+	defer ast.destroy_module(&ast_module)
+	hir_module := checker.check(&ast_module, &diagnostics, &symbols)
+	defer hir.destroy_module(&hir_module)
+
+	wide_symbol := symbol.intern(&symbols, "wide")
+	value_symbol := symbol.intern(&symbols, "value")
+	found_value_i16 := false
+	found_result_i16 := false
+	for function in hir_module.functions {
+		if function.name != wide_symbol {
+			continue
+		}
+		found_result_i16 = function.result == types.I16
+		for local in function.locals {
+			found_value_i16 = found_value_i16 || local.name == value_symbol && local.type == types.I16
+		}
+	}
+
+	testing.expect_value(t, len(diagnostics.items), 0)
+	testing.expect(t, found_result_i16)
+	testing.expect(t, found_value_i16)
+}
+
+@(test)
+undefined_accepts_concrete_runtime_annotations :: proc(t: ^testing.T) {
+	text := `Point :: struct {
+	x i32
+	y i32
+}
+main :: func() void {
+	point Point = undefined
+	pointer @i32 = undefined
+	maybe ?i32 = undefined
+}
+`
+	source_file := source.Source{path="test.bro", text=text}
+	diagnostics := source.init_diagnostics(&source_file)
+	defer source.destroy_diagnostics(&diagnostics)
+	symbols := symbol.init_table()
+	defer symbol.destroy_table(&symbols)
+	stream := lexer.lex(&source_file, &diagnostics, &symbols)
+	defer delete(stream.items)
+	ast_module := parser.parse(&stream, &source_file, &diagnostics)
+	defer ast.destroy_module(&ast_module)
+	hir_module := checker.check(&ast_module, &diagnostics, &symbols)
+	defer hir.destroy_module(&hir_module)
+	ir_module := lower.lower(&hir_module)
+	defer ir.destroy_module(&ir_module)
+
+	main_symbol := symbol.intern(&symbols, "main")
+	fill_count := 0
+	for function, function_index in hir_module.functions {
+		if function.name != main_symbol {
+			continue
+		}
+		for instruction in ir_module.functions[function_index].instructions {
+			fill_count += 1 if instruction.op == .Fill else 0
+		}
+	}
+
+	testing.expect_value(t, len(diagnostics.items), 0)
+	testing.expect_value(t, fill_count, 3)
+}
+
+@(test)
+undefined_rejects_non_declaration_uses_and_unresolved_inference :: proc(t: ^testing.T) {
+	text := `global :: undefined
+main :: func() void {
+	immutable :: undefined
+	typed_immutable int :: undefined
+	unresolved int = undefined
+	existing i32 = 1
+	existing = undefined
+	mismatch int = undefined
+	mismatch = 1
+	mismatch = 1.0
+}
+`
+	source_file := source.Source{path="test.bro", text=text}
+	diagnostics := source.init_diagnostics(&source_file)
+	defer source.destroy_diagnostics(&diagnostics)
+	symbols := symbol.init_table()
+	defer symbol.destroy_table(&symbols)
+	stream := lexer.lex(&source_file, &diagnostics, &symbols)
+	defer delete(stream.items)
+	ast_module := parser.parse(&stream, &source_file, &diagnostics)
+	defer ast.destroy_module(&ast_module)
+	hir_module := checker.check(&ast_module, &diagnostics, &symbols)
+	defer hir.destroy_module(&hir_module)
+
+	immutable_count := 0
+	found_unresolved := false
+	found_assignment := false
+	found_incompatible := false
+	for diagnostic in diagnostics.items {
+		immutable_count += 1 if strings.contains(diagnostic.message, "'undefined' requires a mutable local declaration") else 0
+		found_unresolved = found_unresolved || strings.contains(diagnostic.message, "could not infer a concrete type for local 'unresolved'")
+		found_assignment = found_assignment || strings.contains(diagnostic.message, "'undefined' is only valid as a mutable local declaration initializer")
+		found_incompatible = found_incompatible || strings.contains(diagnostic.message, "cannot implicitly convert f64 to i8")
+	}
+
+	testing.expect(t, immutable_count >= 2)
+	testing.expect(t, found_unresolved)
+	testing.expect(t, found_assignment)
+	testing.expect(t, found_incompatible)
+}
+
 @(test)
 compound_assignment_evaluates_lvalue_once :: proc(t: ^testing.T) {
 	// A compound assignment to an indexed lvalue must compute the element address