broaden type inference from context (first pass)

TODO.md

@@ -198,9 +198,40 @@
       a `float` param accepts an integer-literal argument as f64 (e.g. `f(3)`)
     - a function result is narrowed to the constraint's family
 
-14. broaden type inference to surrounding context
+14. broaden type inference to surrounding context (implemented)
+  - a slot's concrete type is the join of demands reachable from its declaration,
+    flowing backward as well as forward to a fixpoint (the existing global/spec
+    fixpoint in `infer_all`), generalizing milestone 13's forward-only resolution
+  - an "open constant" (a global or local with no concrete annotation plus a compile-time
+    integer initializer) is sign-agnostic until used: a backward demand from any reachable
+    use picks its family/width as long as the value fits, so `A :: 10` followed by
+    `B u16 :: A` resolves both to u16 — the literal's smallest-signed default no longer
+    blocks an unsigned demand; absent any demand it defaults to the smallest signed type
+  - a concrete declared type flows backward through a chain of bare-name references:
+    `X :: 1000; Y int :: X; Z i32 :: Y` resolves X and Y to i32 (previously they stayed
+    at the literal's i16)
+  - locals resolve identically to globals (no scope asymmetry): demands flow through
+    bare-name typed declarations, call arguments (a concrete parameter type demands its
+    argument, e.g. `take_u16(a)`), and returns — including from inside a function body
+    back onto a referenced global
+  - demands flow only through bare names; they do not cross arithmetic or other operators,
+    nor back across a call's result (the result-to-argument direction is milestone 14.5)
+  - a non-fitting or family-conflicting demand is not applied (first demand wins); the
+    genuine mismatch then surfaces as the usual boundary coercion error at the use
+    (e.g. `C u8 :: BIG` where `BIG :: 100000`)
 
-15. add slice-by-range
+14.5. backward type-demand propagation through call boundaries (deferred)
+  - a callee's result/return demand flows back through the function body to constrain
+    the caller's arguments, so `R u32 :: echo(A)` (with `echo :: func(p int) int`)
+    resolves A to u32 instead of erroring at the call's result coercion
+  - requires reversing the per-call data flow: a specialization's argument types
+    (`spec.args`) become outputs to solve, not just inputs — a new back-edge threaded
+    through every call site and the specialization fixpoint
+  - only meaningful on top of milestone 14's open constants
+
+15. broaden type inference to infer type of declaration based on arithmetic expressions too
+
+16. add slice-by-range
   - allow the use of a range in slice expressions:
     ```
     excl_range range :: 0..10
@@ -210,6 +241,22 @@
     some_arr[incl_range] # slice by named inclusive range
     ```
 
+17. for if statements, allow `if (cond) one-line statement` (instead of forcing either `if (cond) { block }` or `if cond { block }`)
+  - if statements without a bracketed body must enclose the condition in parentheses
+
+18. add `defer` statement (inspired by zig)
+
+19. multi-line strings (see below)
+
+20. unions and tagged unions
+
+21. match statements with tagged unions payload unwrapping
+
+22. dynamic heap allocation
+  - see below for direction
+  - notes below are too big in scope for a first pass and the language is not mature enough to support it yet
+  - this first pass should focus on just basic heap allocation, so we have something to work with
+
 ## A word on multi-unwrap
 
 Unwrap multiple optionals with `and`. This **short-circuits**: if the first optional is none, subsequent expressions are not evaluated.
@@ -276,7 +323,7 @@ This rule keeps the grammar simple and forces clarity at the call site — no pr
 
 For-loop captures are immutable and scoped to the loop body. Sequence index captures are `usize`. Pointer capture uses `|@item|`; arrays must be passed by pointer (for example `&items`), while slices can be used directly. Sentinel elements are not included in iteration.
 
-# A word on distinct types
+## A word on distinct types
 
 Distinct types are considered distinct from their backing type. They do not implicitly coerce to their backing type.
 
@@ -288,7 +335,7 @@ UserID :: distinct u32
 my_id UserID :: UserID(42) # value must have the exact backing type
 ```
 
-# A word on enums
+## A word on enums
 
 ```
 # standard enums
@@ -347,3 +394,311 @@ value native.Imported_Enum :: native.IMPORTED_ENUM_VALUE
 
 The imported enum type is an alias of its target-selected C integer backing, and imported
 enumerators are package-level constants.
+
+## A word on multi-line strings
+
+Multi-line strings use the `` ` `` character to mark each line. Content starts immediately after the backtick. Newlines between lines are implicit.
+
+```
+config =
+    `# Database configuration
+    `host = localhost
+    `port = 5432
+    `
+    `[server]
+    `address = 0.0.0.0
+```
+
+Key properties:
+
+* Content begins immediately after `` ` `` 
+* Newlines are automatically inserted between lines
+* Empty `` ` `` produces a blank line
+* No escape sequence processing (raw content)
+* No trailing newline after the last line
+
+Only the leading `` ` `` is special; the rest is treated as raw content.
+
+If you need a trailing newline, add an empty line at the end:
+
+```
+# No trailing newline
+msg =
+    `hello
+    `world
+
+# With trailing newline
+msg =
+    `hello
+    `world
+    `
+```
+
+Mixing multi-line strings with inline strings (using concatenation):
+
+```
+message = 
+    "Header:\t" ++
+    `more content here
+    `even more content
+    `
+    ++ "Footer"
+```
+
+Formatting alternative (purely aesthetics/preference, no effect on program):
+
+```
+message =  "Header:\t" ++
+           `more content here
+           `even more content
+           `
+           ++ "Footer"
+```
+
+## A word on memory allocation
+
+(NOTE THAT SYNTAX MAY NOT MATCH BROLANG EXACTLY AND SHOULD BE TAKEN WITH A GRAIN OF SALT - INSPIRATION ONLY)
+(FURTHER, EXAMPLES ASSUME ARGUMENTS WITH DEFAULT VALUES AND COMPTIME POLYMORPHISM IN THE FORM OF GENERIC TYPE PARAMETERS - MONOMORPHISED)
+
+Memory allocation in Brolang is designed to be **explicit but not verbose**. We reject the dogma that global state is inherently evil — allocators are a cross-cutting concern that nearly every function needs, making them a perfect candidate for sensible defaults.
+
+### Philosophy
+
+```
+┌─────────────────────────────────────────────────────────────┐
+│  DESIGN PRINCIPLES                                          │
+│                                                             │
+│  1. No hidden magic: allocation calls are visible           │
+│  2. Sensible defaults: thread-local heap for common cases   │
+│  3. Explicit override: custom allocators when needed        │
+│  4. Build-mode aware: different behavior for debug/release  │
+│  5. Immutable defaults: no "action at a distance" bugs      │
+│  6. Escaping allocations: caller provides allocator         │
+└─────────────────────────────────────────────────────────────┘
+```
+
+### The Default Heap Allocator
+
+Brolang provides a **thread-local global heap allocator** that is:
+
+* Determined at compile time by build mode
+* **Immutable at runtime** — cannot be reconfigured
+
+```
+import "std/mem/heap"
+
+process :: func(input []u8) u64 {
+    # heap used for internal temporary work — does not escape
+    temp := heap.alloc(u8, size: input.len * 2)
+    defer heap.free(temp)
+
+    # ... work with temp ...
+
+    return compute_hash(temp)  # only the result escapes, not the allocation
+}
+```
+
+The behavior of `heap` depends on build mode:
+
+| Build Mode | Allocator Behavior |
+| -- | -- |
+| Debug | Tracking allocator with leak detection |
+| Release | Fast allocator, zero overhead |
+| ReleaseSafe | Bounds-checking allocator |
+
+This is configured at compile time. You cannot change which allocator `heap` uses at runtime. This is intentional — it prevents bugs where memory allocated with one allocator is freed with another.
+
+### The Escaping Allocation Rule
+
+**If a function heap-allocates memory that escapes its scope — whether via the return value or via writes through mutable parameters — the function must accept an allocator parameter.** The presence of an allocator parameter is the contract that says "heap memory escapes here, and you're responsible for it."
+
+This rule makes ownership transfer visible at the function signature level. The caller never needs to read the function's implementation to know whether heap cleanup is involved:
+
+```
+import "std/mem"
+import "std/mem/heap"
+
+# Allocation escapes via return value — requires allocator
+duplicate :: func(input []u8, allocator @mem.Allocator) []u8 {
+    result := allocator.alloc(u8, size: input.len)
+    mem.copy(result, input)
+    return result  # caller manages this memory
+}
+
+# Allocation escapes via mutable parameter — requires allocator
+init :: func(obj: @mut MyStruct, allocator: @mem.Allocator) void {
+    obj.buffer = allocator.alloc(u8, size: 100)
+    # caller now knows heap memory was written into obj
+}
+
+# No allocation escapes — no allocator needed
+process :: func(input: []u8) u64 {
+    temp := heap.alloc(u8, size: input.len)
+    defer heap.free(temp)
+    # ... work with temp ...
+    return compute_hash(temp)
+}
+
+# No heap allocation at all — no allocator needed
+reset :: func(obj: @mut MyStruct) void {
+    obj.count = 0
+}
+
+main :: func() void {
+    data := duplicate("hello", heap)
+    defer heap.free(data)
+
+    mut obj := MyStruct{ ... }
+    init(&obj, heap)
+    defer heap.free(obj.buffer)
+}
+```
+
+**Why this matters:**
+
+* Without the rule, a function like `init(obj: @mut MyStruct) void` is ambiguous — did it heap-allocate into `obj`, or just set some fields to stack/static data? The caller has no way to know without reading the implementation.
+* With the rule, the allocator parameter is a clear signal: "this function produces heap memory that outlives its scope, and you are responsible for cleaning it up."
+* Internal allocations (temporary buffers, scratch space) use `heap` directly and are freed before the function returns. No allocator parameter needed, no burden on the caller.
+
+**The compiler enforces this rule.** If a function heap-allocates memory that escapes without accepting an allocator parameter, the compiler emits an error.
+
+### Why Immutable Defaults?
+
+Consider what would happen if you could reconfigure the default allocator:
+
+```
+# ❌ THIS IS NOT ALLOWED (and doesn't exist in Brolang)
+mem.heap_set(my_custom_heap)
+
+# Somewhere else in the codebase...
+data := heap.alloc(u8, size: 100)
+
+# Later, someone changes it again...
+mem.heap_set(different_heap)
+
+# Now who frees `data`? With which allocator?
+heap.free(data)  # 💥 Wrong allocator - undefined behavior!
+```
+
+This is "action at a distance" — the behavior of `heap.free()` depends on what some unrelated code did earlier. By making `heap` immutable, Brolang guarantees:
+
+**Whatever you allocate with, you free with.**
+
+### Custom Allocators
+
+For specialized needs, you create explicit allocator instances. These are not global — you manage their lifetime and pass them where needed.
+
+**Arena Allocator**: Fast bump allocation, bulk deallocation:
+
+```
+import "std/mem"
+import "std/mem/heap"
+
+process_file :: func(path: []u8, allocator: @mem.Allocator) !Data {
+    # arena manages its own backing memory via heap
+    arena := mem.Arena.init(heap, capacity: mem.megabytes(1))
+    defer arena.deinit()
+
+    # all temporary allocations from arena (fast bump allocation)
+    file_contents := arena.alloc(u8, size: file_size)
+    parsed := arena.alloc(ParsedData)         # size defaults to 1
+    tokens := arena.alloc(Token, size: 1000)
+
+    # ... process ...
+
+    # escaping allocation uses the caller's allocator
+    result := allocator.create(Data)
+    mem.copy(result, parsed)
+
+    return result
+    # arena.deinit() frees all arena memory — no individual frees needed
+}
+```
+
+**Pool Allocator**: O(1) fixed-size allocation, no fragmentation:
+
+```
+import "std/mem"
+import "std/mem/heap"
+
+EntitySystem :: struct {
+    pool: mem.Pool(Entity),
+}
+
+init_entities :: func(allocator: @mem.Allocator) EntitySystem {
+    return EntitySystem{
+        pool = mem.Pool(Entity).init(allocator, capacity: 10_000),
+    }
+}
+
+spawn :: func(sys: @mut EntitySystem) @Entity {
+    return sys.pool.alloc()  # O(1), no fragmentation
+}
+
+despawn :: func(sys: @mut EntitySystem, entity: @Entity) void {
+    sys.pool.free(entity)  # returned to pool for reuse
+}
+```
+
+### Passing Allocators to Functions
+
+As described in the escaping allocation rule, when a function heap-allocates memory that escapes its scope, it must accept an allocator parameter. The caller decides which allocator to use:
+
+```
+import "std/mem"
+
+# Function that uses caller's allocator
+parse :: func(input: []u8, allocator: @mem.Allocator) !ParseResult {
+    buffer := allocator.alloc(u8, size: input.len)
+    defer allocator.free(buffer)
+    
+    # ... parse into buffer ...
+    
+    result := allocator.alloc(ParseResult)  # size defaults to 1
+    return result
+}
+
+# Caller decides which allocator to use
+main :: func() void {
+    # use an arena for this parsing work
+    arena := mem.Arena.init(heap, capacity: mem.kilobytes(64))
+    defer arena.deinit()
+    result := parse(input, &arena) catch |err| {
+        # handle error
+    }
+
+    # or use a pool
+    pool := mem.Pool(ParseResult).init(capacity: 100)
+    defer pool.deinit()
+    result := parse(input, &pool) catch |err| {
+        # handle error
+    }
+}
+```
+
+### Memory Allocation Summary
+
+| What | How | When to Use |
+| -- | -- | -- |
+| `heap.alloc(T, size: n)` | Thread-local global | General purpose, 90% of cases |
+| `heap.create(T)` | Thread-local global | Allocate single item |
+| `allocator.alloc(T, size: n)` | Caller-provided | Escaping allocations (returned or written to caller's data) |
+| `arena.alloc(T, size: n)` | Explicit instance | Temporary/scoped work, bulk free |
+| `pool.alloc()` | Explicit instance | Many same-sized objects, O(1) |
+
+Note that `heap` satisfies the `Allocator` interface, so callers can pass `heap` as the allocator argument when they don't need a specialized allocator — which is most of the time.
+
+**The golden rule:** Allocate and free with the same allocator. The type system helps enforce this — memory from `heap` can only be freed with `heap`, memory from your arena can only be freed with that arena. The escaping allocation rule ensures the caller always knows which allocator was used.
+
+### Compared to Other Languages
+
+| Language | Approach | Brolang's Advantage |
+| -- | -- | -- |
+| C | Hidden malloc, easy to mismatch | Explicit allocator at call site |
+| C++ | Allocator templates, complex | Simple, no template complexity |
+| Rust | Explicit everywhere, verbose | Sensible defaults reduce noise |
+| Zig | Allocator parameter threading | Only required for escaping allocations, not internal work |
+| Odin | Hidden context parameter | Fully transparent, nothing hidden |
+| Go | Hidden GC | Explicit control, no GC pauses |
+
+Brolang sits in a sweet spot: explicit enough to always know what's happening, convenient enough that you don't drown in boilerplate.

compiler/checker/checker.odin

@@ -38,6 +38,12 @@ Infer_Local :: struct {
 	declared:  types.Type,
 	statement: ast.Stmt_Id,
 	mutable:   bool,
+	// open_const marks a local whose initializer is a compile-time integer with no
+	// concrete annotation: like an open-constant global, it is sign-agnostic until a
+	// backward demand from a use picks its family/width (see merge_local_demand).
+	open_const:  bool,
+	const_value: i128,
+	demanded:    bool,
 }
 
 Build_Local :: struct {
@@ -105,6 +111,14 @@ Checker :: struct {
 	global_index:         []Global_Index_Entry,
 	import_index:         []Import_Index_Entry,
 	global_types:         []types.Type,
+	// Backward type-demand state for open-constant globals (milestone 14). global_demands
+	// accumulates demands reachable from any use (other globals' initializers and function
+	// bodies); global_demands_dirty lets a demand pushed from a function body re-trigger the
+	// inference fixpoint.
+	global_demands:       []types.Type,
+	global_open_const:    []bool,
+	global_const_value:   []i128,
+	global_demands_dirty: bool,
 	external_global_canonical: []ast.Global_Id,
 	external_global_diagnostics: []source.Diagnostic_Id,
 	constants:            []Constant,
@@ -1088,11 +1102,12 @@ infer_nested_expr :: proc(
 	pkg: ast.Package_Id,
 	file: ast.File_Id,
 	demanded: ^[dynamic]Spec_Id,
+	local_types: []types.Type = nil,
 ) -> types.Type {
 	outer := checker.infer_stack
 	checker.infer_stack = nil
 	checker.infer_stack.allocator = checker.allocator
-	result := infer_expr(checker, expr_id, locals, pkg, file, demanded)
+	result := infer_expr(checker, expr_id, locals, pkg, file, demanded, local_types)
 	delete(checker.infer_stack)
 	checker.infer_stack = outer
 	return result
@@ -1105,21 +1120,22 @@ infer_compound_expr :: proc(
 	pkg: ast.Package_Id,
 	file: ast.File_Id,
 	demanded: ^[dynamic]Spec_Id,
+	local_types: []types.Type = nil,
 ) -> types.Type {
 	store := &checker.module.types
 	#partial switch expr.kind {
 	case .Bool:
 		return types.BOOL
 	case .Not:
-		_ = infer_nested_expr(checker, expr.left, locals, pkg, file, demanded)
+		_ = infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
 		return types.BOOL
 	case .Eq, .Ne, .Lt, .Le, .Gt, .Ge, .And, .Or:
-		_ = infer_nested_expr(checker, expr.left, locals, pkg, file, demanded)
+		_ = infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
-		_ = infer_nested_expr(checker, expr.right, locals, pkg, file, demanded)
+		_ = infer_nested_expr(checker, expr.right, locals, pkg, file, demanded, local_types)
 		return types.BOOL
 	case .Range:
-		left := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded)
+		left := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
-		right := infer_nested_expr(checker, expr.right, locals, pkg, file, demanded)
+		right := infer_nested_expr(checker, expr.right, locals, pkg, file, demanded, local_types)
 		left_const := eval_constant(checker, expr.left)
 		right_const := eval_constant(checker, expr.right)
 		child := types.INVALID
@@ -1139,7 +1155,7 @@ infer_compound_expr :: proc(
 	case .Array:
 		element := types.INVALID
 		for arg in expr.args {
-			actual := infer_nested_expr(checker, arg, locals, pkg, file, demanded)
+			actual := infer_nested_expr(checker, arg, locals, pkg, file, demanded, local_types)
 			if !types.is_valid(element) {
 				element = actual
 			} else if !types.equal(element, actual) {
@@ -1157,25 +1173,25 @@ infer_compound_expr :: proc(
 	case .Enum_Literal:
 		return types.INVALID
 	case .Address:
-		child := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded)
+		child := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
 		return types.pointer(store, child, false, false)
 	case .Deref:
-		value := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded)
+		value := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
 		return types.child_type(value, store) if types.is_pointer(value, store) else types.INVALID
 	case .Index:
-		value := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded)
+		value := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
-		_ = infer_nested_expr(checker, expr.right, locals, pkg, file, demanded)
+		_ = infer_nested_expr(checker, expr.right, locals, pkg, file, demanded, local_types)
 		item, ok := types.container(value, store)
 		return item.child if ok else types.INVALID
 	case .Slice:
-		value := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded)
+		value := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
 		item, ok := types.container(value, store)
 		if !ok || item.kind == .Pointer {
 			return types.INVALID
 		}
 		for bound in expr.args {
 			if bound != ast.INVALID_EXPR {
-				_ = infer_nested_expr(checker, bound, locals, pkg, file, demanded)
+				_ = infer_nested_expr(checker, bound, locals, pkg, file, demanded, local_types)
 			}
 		}
 		preserve := item.has_sentinel && expr.args[1] == ast.INVALID_EXPR
@@ -1185,7 +1201,7 @@ infer_compound_expr :: proc(
 			_, member_ok := find_enum_member(checker, enum_type, expr.name)
 			return enum_type if member_ok else types.INVALID
 		}
-		value := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded)
+		value := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
 		field_name := symbol_text(checker, expr.name)
 		item, has_item := types.container(value, store)
 		if has_item && (item.kind == .Array || item.kind == .Slice) {
@@ -1203,21 +1219,21 @@ infer_compound_expr :: proc(
 		_, field, ok := find_struct_field(checker, value, expr.name)
 		return field.type if ok else types.INVALID
 	case .Unwrap:
-		value := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded)
+		value := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
 		return types.child_type(value, store) if types.is_optional(value, store) else types.INVALID
 	case .Orelse:
-		value := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded)
+		value := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
-		_ = infer_nested_expr(checker, expr.right, locals, pkg, file, demanded)
+		_ = infer_nested_expr(checker, expr.right, locals, pkg, file, demanded, local_types)
 		return types.child_type(value, store) if types.is_optional(value, store) else types.INVALID
 	case .Struct_Literal:
 		for keyed in expr.args {
-			_ = infer_nested_expr(checker, checker.ast_module.exprs[keyed].left, locals, pkg, file, demanded)
+			_ = infer_nested_expr(checker, checker.ast_module.exprs[keyed].left, locals, pkg, file, demanded, local_types)
 		}
 		target_pkg, available := expr_package(checker, expr, pkg, file)
 		value := types.find_named(store, u32(target_pkg), u32(expr.name)) if available else types.INVALID
 		return types.resolve_alias(value, store)
 	case .Keyed:
-		return infer_nested_expr(checker, expr.left, locals, pkg, file, demanded)
+		return infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
 	case:
 		return types.INVALID
 	}
@@ -1230,6 +1246,7 @@ infer_expr :: proc(
 	pkg := ast.Package_Id(0),
 	file := ast.File_Id(0),
 	demanded: ^[dynamic]Spec_Id = nil,
+	local_types: []types.Type = nil,
 ) -> types.Type {
 	stack := checker.infer_stack
 	clear_dynamic_array(&stack)
@@ -1281,7 +1298,7 @@ infer_expr :: proc(
 			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)
+				last = infer_compound_expr(checker, expr, locals, pkg, file, demanded, local_types)
 				_ = pop(&stack)
 			case .Name:
 				last = types.INVALID
@@ -1349,7 +1366,7 @@ infer_expr :: proc(
 				append(&stack, Infer_Frame{expr=expr.left, template=ast.INVALID_FUNCTION})
 			case .Call:
 				if expr.left != ast.INVALID_EXPR {
-					callee_type := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded)
+					callee_type := infer_nested_expr(checker, expr.left, locals, pkg, file, demanded, local_types)
 					_, function_item, function_type, ok := types.function_pointer(callee_type, &checker.module.types)
 					if !ok {
 						last = types.INVALID
@@ -1464,6 +1481,13 @@ infer_expr :: proc(
 				}
 			}
 			function := checker.ast_module.functions[frame.template]
+			// A bare-name argument passed to a concrete (non-constraint) parameter pushes
+			// that parameter type back onto the argument's slot, so an open constant adopts
+			// 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)
+			}
 			if valid_call_arity(function, len(expr.args)) &&
 			   can_specialize(checker, function, stack[frame_index].args) {
 				spec := INVALID_SPEC
@@ -1612,7 +1636,7 @@ infer_statements :: proc(
 			declared_local := resolve_inferred_array(checker, type_from_syntax(statement.type), statement.expr)
 			value_type := types.INVALID
 			if !is_undefined_expr(checker, statement.expr) {
-				value_type = infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded)
+				value_type = infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded, local_types)
 			}
 			if is_runtime_type(checker, declared_local) {
 				value_type = declared_local
@@ -1621,19 +1645,33 @@ infer_statements :: proc(
 				// build_block reports). FLOAT defaults integers to f64.
 				value_type = types.constraint_target(declared_local, value_type, &checker.module.types)
 			}
+			open := false
+			const_val := i128(0)
+			if !is_runtime_type(checker, declared_local) && !is_undefined_expr(checker, statement.expr) {
+				constant := eval_constant(checker, statement.expr)
+				if constant.kind == .Value && fits_i64(constant.value) {
+					open = true
+					const_val = constant.value
+				}
+			}
 			local := Infer_Local{
 				name=statement.name,
 				type=value_type,
 				declared=declared_local,
 				statement=statement_id,
 				mutable=!statement.immutable,
+				open_const=open,
+				const_value=const_val,
 			}
 			append(locals, local)
 			record_infer_local_type(local, local_types)
+			// A typed/constraint declaration initialized by a bare name pushes its resolved
+			// type backward onto that name (mirrors the `Y int :: X; Z i32 :: Y` global chain).
+			record_demand(checker, statement.expr, value_type, locals^[:], local_types, pkg, file)
 		case .Assignment:
-			value_type := infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded)
+			value_type := infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded, local_types)
 			if statement.target != ast.INVALID_EXPR {
-				_ = infer_expr(checker, statement.target, locals^[:], pkg, file, demanded)
+				_ = infer_expr(checker, statement.target, locals^[:], pkg, file, demanded, local_types)
 				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 &&
@@ -1648,16 +1686,23 @@ infer_statements :: proc(
 				}
 			}
 		case .Expression:
-			_ = infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded)
+			_ = infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded, local_types)
 		case .Return:
 			if statement.expr != ast.INVALID_EXPR {
-				returned := infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded)
+				returned := infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded, local_types)
 				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)
+							// Open constants can adopt the result type cross-family; other
+							// locals widen within family as before.
+							if !merge_local_demand(checker, &locals^[local_index], result_hint, local_types) {
+								_ = merge_infer_local_type(checker, &locals^[local_index], result_hint, local_types)
+							}
 							returned = result_hint
+						} else {
+							// `return G` for a global const: demand the result type onto it.
+							record_demand(checker, statement.expr, result_hint, locals^[:], local_types, pkg, file)
 						}
 					}
 				}
@@ -1674,7 +1719,7 @@ infer_statements :: proc(
 				flatten_conditional_unwrap_operands(checker.ast_module, statement.expr, &operands)
 				operand_types := make([]types.Type, len(operands), checker.allocator)
 				for operand, index in operands {
-					operand_types[index] = infer_expr(checker, operand, locals^[:], pkg, file, demanded)
+					operand_types[index] = infer_expr(checker, operand, locals^[:], pkg, file, demanded, local_types)
 				}
 				capture_start := len(locals^)
 				for capture, index in statement.captures {
@@ -1689,7 +1734,7 @@ infer_statements :: proc(
 					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_expr(checker, statement.guard, locals^[:], pkg, file, demanded, local_types)
 				}
 				infer_statements(checker, statement.body, locals, local_types, pkg, file, demanded, result, result_hint)
 				resize(locals, capture_start)
@@ -1697,19 +1742,19 @@ infer_statements :: proc(
 				delete(operand_types, checker.allocator)
 				delete(operands)
 			} else {
-				_ = infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded)
+				_ = infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded, local_types)
 				infer_statements(checker, statement.body, locals, local_types, pkg, file, demanded, result, result_hint)
 				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_expr(checker, statement.expr, locals^[:], pkg, file, demanded, local_types)
 			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, local_types, pkg, file, demanded, result, result_hint)
 			}
 		case .For:
-			iterable_type := infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded)
+			iterable_type := infer_expr(checker, statement.expr, locals^[:], pkg, file, demanded, local_types)
 			capture_start := len(locals^)
 			capture_type := types.INVALID
 			if types.is_range(iterable_type, &checker.module.types) {
@@ -1797,11 +1842,158 @@ merge_inferred_type :: proc(store: ^types.Store, current: ^types.Type, inferred:
 	return false
 }
 
+// root_demand_target returns the global that an initializer pushes a backward type
+// demand onto: when the initializer's root expression is a bare name referencing a
+// global (e.g. `Z i32 :: Y`). Returns INVALID_GLOBAL for any other shape — demands
+// deliberately do not flow through arithmetic, calls, or other operators (that is L3).
+root_demand_target :: proc(checker: ^Checker, expr_id: ast.Expr_Id, pkg: ast.Package_Id, file: ast.File_Id) -> ast.Global_Id {
+	if expr_id == ast.INVALID_EXPR || int(expr_id) >= len(checker.ast_module.exprs) {
+		return ast.INVALID_GLOBAL
+	}
+	expr := checker.ast_module.exprs[expr_id]
+	if expr.kind != .Name {
+		return ast.INVALID_GLOBAL
+	}
+	target_pkg, available := expr_package(checker, expr, pkg, file)
+	if !available {
+		return ast.INVALID_GLOBAL
+	}
+	return find_global(checker, expr.name, target_pkg)
+}
+
+// merge_open_const_demand records a concrete integer demand onto an open-constant
+// global's slot. The constant is sign-agnostic until used, so it may adopt any
+// integer family/width whose range holds its value (first demand wins; later demands
+// may only widen within the chosen family). Non-integer or non-fitting demands are
+// ignored, leaving the constant to default and the genuine mismatch to surface at the
+// use's boundary coercion.
+merge_open_const_demand :: proc(checker: ^Checker, slot: ^types.Type, demand: types.Type, value: i128) -> bool {
+	if !types.is_concrete_integer(demand) || !fits_integer_type(value, demand, checker.target) {
+		return false
+	}
+	if !is_runtime_type(checker, slot^) {
+		slot^ = demand
+		return true
+	}
+	if types.equal(slot^, demand) {
+		return false
+	}
+	merged := types.widest(slot^, demand)
+	if types.is_concrete_scalar(merged) && !types.equal(slot^, merged) {
+		slot^ = merged
+		return true
+	}
+	return false
+}
+
+// merge_global_demand routes a concrete demand onto a global's slot: open constants
+// adopt any fitting family, other referents widen within family. Sets a dirty flag so
+// a demand pushed from a function body re-triggers the inference fixpoint.
+merge_global_demand :: proc(checker: ^Checker, global: ast.Global_Id, demand: types.Type) -> bool {
+	index := int(global)
+	if index < 0 || index >= len(checker.global_demands) {
+		return false
+	}
+	changed: bool
+	if checker.global_open_const[index] {
+		changed = merge_open_const_demand(checker, &checker.global_demands[index], demand, checker.global_const_value[index])
+	} else {
+		changed = merge_inferred_type(&checker.module.types, &checker.global_demands[index], demand)
+	}
+	checker.global_demands_dirty = checker.global_demands_dirty || changed
+	return changed
+}
+
+// merge_local_demand records a concrete integer demand onto an open-constant local.
+// Like an open-constant global it adopts any integer family/width whose range holds its
+// value (gated by its constraint family if it has one); the first demand replaces the
+// literal's signed default, later demands may only widen within the chosen family.
+merge_local_demand :: proc(checker: ^Checker, local: ^Infer_Local, demand: types.Type, local_types: []types.Type) -> bool {
+	if !local.open_const || !types.is_concrete_integer(demand) {
+		return false
+	}
+	if types.is_constraint(local.declared) &&
+	   !types.constraint_accepts(local.declared, demand, &checker.module.types) {
+		return false
+	}
+	if !fits_integer_type(local.const_value, demand, checker.target) {
+		return false
+	}
+	if !local.demanded {
+		local.type = demand
+		local.demanded = true
+		record_infer_local_type(local^, local_types)
+		return true
+	}
+	if types.equal(local.type, demand) {
+		return false
+	}
+	merged := types.widest(local.type, demand)
+	if types.is_concrete_scalar(merged) && !types.equal(local.type, merged) {
+		local.type = merged
+		record_infer_local_type(local^, local_types)
+		return true
+	}
+	return false
+}
+
+// record_demand pushes a concrete type demand onto the slot of a bare-name expression
+// (a typed declaration's initializer, a call argument, a return value). When the name
+// resolves to an open-constant local or global, that slot adopts the demand; any other
+// shape is ignored — demands flow only through bare names, never through arithmetic or
+// across a call's result (the latter is milestone 14.5).
+record_demand :: proc(
+	checker: ^Checker,
+	expr_id: ast.Expr_Id,
+	demand: types.Type,
+	locals: []Infer_Local,
+	local_types: []types.Type,
+	pkg: ast.Package_Id,
+	file: ast.File_Id,
+) {
+	if !is_runtime_type(checker, demand) ||
+	   expr_id == ast.INVALID_EXPR || int(expr_id) >= len(checker.ast_module.exprs) {
+		return
+	}
+	expr := checker.ast_module.exprs[expr_id]
+	if expr.kind != .Name {
+		return
+	}
+	if !symbol.is_valid(expr.qualifier) {
+		if index, ok := find_infer_local_index(locals, expr.name); ok {
+			_ = merge_local_demand(checker, &locals[index], demand, local_types)
+			return
+		}
+	}
+	target_pkg, available := expr_package(checker, expr, pkg, file)
+	if !available {
+		return
+	}
+	global := find_global(checker, expr.name, target_pkg)
+	if global != ast.INVALID_GLOBAL {
+		_ = merge_global_demand(checker, global, demand)
+	}
+}
+
 infer_all :: proc(checker: ^Checker) {
+	// An "open constant" global has no concrete declared type and a compile-time
+	// integer initializer. Its slot stays sign-agnostic so a backward demand from any
+	// reachable use can pick its family/width; absent a demand it defaults to the
+	// smallest signed type (legacy behaviour). Demands accumulate in global_demands so
+	// the default never blocks a later cross-family (e.g. unsigned) demand.
 	for global, index in checker.ast_module.globals {
 		declared := type_from_syntax(global.type)
 		if is_runtime_type(checker, declared) {
 			checker.global_types[index] = declared
+			continue
+		}
+		if global.external {
+			continue
+		}
+		constant := eval_constant(checker, global.expr)
+		if constant.kind == .Value && fits_i64(constant.value) {
+			checker.global_open_const[index] = true
+			checker.global_const_value[index] = constant.value
 		}
 	}
 
@@ -1812,7 +2004,29 @@ infer_all :: proc(checker: ^Checker) {
 
 	for {
 		changed := false
+		checker.global_demands_dirty = false
 		spec_count := len(checker.specs)
+
+		// Backward demands: a global whose initializer's root is a bare name referencing
+		// another global pushes its own (declared or already-resolved) type onto that
+		// referent. Open constants adopt any fitting family; other referents widen only.
+		for global, index in checker.ast_module.globals {
+			if global.external {
+				continue
+			}
+			demand := checker.global_types[index]
+			if !is_runtime_type(checker, demand) {
+				continue
+			}
+			target := root_demand_target(checker, global.expr, global.pkg, global.file)
+			if target != ast.INVALID_GLOBAL {
+				merge_global_demand(checker, target, demand)
+			}
+		}
+
+		// Forward / resolution. infer_expr runs for every non-external global (even
+		// concrete-typed ones) for its side effect of specializing called functions and
+		// recording demands from call arguments in their initializers.
 		for global, index in checker.ast_module.globals {
 			if global.external {
 				continue
@@ -1821,8 +2035,24 @@ infer_all :: proc(checker: ^Checker) {
 			if is_runtime_type(checker, type_from_syntax(global.type)) {
 				continue
 			}
-			changed = merge_inferred_type(&checker.module.types, &checker.global_types[index], inferred) || changed
+			if is_runtime_type(checker, checker.global_demands[index]) {
+				// A backward demand is authoritative; assign directly (it may cross the
+				// signed/unsigned family that widening would reject).
+				if !types.equal(checker.global_types[index], checker.global_demands[index]) {
+					checker.global_types[index] = checker.global_demands[index]
+					changed = true
+				}
+			} else if checker.global_open_const[index] {
+				resolved := types.smallest_signed_for_literal(i64(checker.global_const_value[index]))
+				if !types.equal(checker.global_types[index], resolved) {
+					checker.global_types[index] = resolved
+					changed = true
+				}
+			} else {
+				changed = merge_inferred_type(&checker.module.types, &checker.global_types[index], inferred) || changed
+			}
 		}
+
 		for index := 0; index < len(checker.specs); index += 1 {
 			id := spec_id(index)
 			inferred := infer_spec_result(checker, id)
@@ -1831,6 +2061,11 @@ infer_all :: proc(checker: ^Checker) {
 		if len(checker.specs) != spec_count {
 			changed = true
 		}
+		// A demand pushed onto a global from inside a function body (via the spec loop)
+		// is picked up by the next pass's resolution, so keep iterating for it.
+		if checker.global_demands_dirty {
+			changed = true
+		}
 		if !changed {
 			break
 		}
@@ -3487,9 +3722,19 @@ build_block :: proc(
 		switch statement.kind {
 		case .Declaration:
 			declared := resolve_inferred_array(checker, type_from_syntax(statement.type), statement.expr)
+			// Adopt the type inference resolved for this local when the declaration has no
+			// concrete annotation (a constraint, `undefined`, or an un-annotated open
+			// integer constant): the slot may have absorbed a backward demand (e.g. `a :: 10`
+			// built as u16 after `take_u16(a)`). Gated to compile-time integer constants so
+			// strings/arrays/pointers keep their own initializer type.
+			open_const_decl := !is_runtime_type(checker, declared) && !is_undefined_expr(checker, statement.expr)
+			if open_const_decl {
+				constant := eval_constant(checker, statement.expr)
+				open_const_decl = constant.kind == .Value && fits_i64(constant.value)
+			}
 			if statement_id != ast.INVALID_STMT && int(statement_id) < len(ctx.local_types) &&
 			   is_runtime_type(checker, ctx.local_types[statement_id]) &&
-			   (types.is_constraint(declared) || is_undefined_expr(checker, statement.expr)) {
+			   (types.is_constraint(declared) || is_undefined_expr(checker, statement.expr) || open_const_decl) {
 				declared = ctx.local_types[statement_id]
 			}
 			// A still-unresolved constraint means the initializer's numeric
@@ -4352,6 +4597,15 @@ build_globals :: proc(checker: ^Checker) {
 		expected := types.INVALID
 		if is_runtime_type(checker, declared) {
 			expected = declared
+		} else if constant := eval_constant(checker, global.expr);
+		   constant.kind == .Value && fits_i64(constant.value) &&
+		   is_runtime_type(checker, checker.global_types[global_index]) {
+			// Open constant: build the initializer against the type inference resolved
+			// for this slot, so it adopts its demanded/defaulted type (e.g. `A :: 10`
+			// built as u16 when a use demanded u16). Gated to compile-time values fitting
+			// i64 — exactly the infer-side open-constant condition — so out-of-range
+			// constants keep their original "exceeds signed i64 range" diagnostic.
+			expected = checker.global_types[global_index]
 		}
 		expr := build_expr(checker, global.expr, nil, &dependencies, &calls, expected, global.pkg, global.file)
 		global_type := checker.global_types[global_index]
@@ -4645,6 +4899,9 @@ check :: proc(
 	checker.cycle_stack.allocator = allocator
 	build_symbol_indexes(&checker)
 	checker.global_types = make([]types.Type, len(ast_module.globals), allocator)
+	checker.global_demands = make([]types.Type, len(ast_module.globals), allocator)
+	checker.global_open_const = make([]bool, len(ast_module.globals), allocator)
+	checker.global_const_value = make([]i128, len(ast_module.globals), allocator)
 	checker.external_global_canonical = make([]ast.Global_Id, len(ast_module.globals), allocator)
 	checker.external_global_diagnostics = make([]source.Diagnostic_Id, len(ast_module.globals), allocator)
 	for &canonical in checker.external_global_canonical {
@@ -4667,6 +4924,9 @@ check :: proc(
 		delete(checker.global_index, allocator)
 		delete(checker.import_index, allocator)
 		delete(checker.global_types, allocator)
+		delete(checker.global_demands, allocator)
+		delete(checker.global_open_const, allocator)
+		delete(checker.global_const_value, allocator)
 		delete(checker.external_global_canonical, allocator)
 		delete(checker.external_global_diagnostics, allocator)
 		delete(checker.constants, allocator)

compiler_tests.odin

@@ -6427,3 +6427,205 @@ main :: func() void {
 	}
 	testing.expect_value(t, len(diagnostics.items), 0)
 }
+
+@(test)
+contextual_inference_resolves_signed_const_chain :: proc(t: ^testing.T) {
+	text := `X :: 1000
+Y int :: X
+Z i32 :: Y
+main :: func() void {}
+`
+	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)
+
+	// The concrete i32 on Z flows backward through Y to the open constant X, so all
+	// three resolve to i32 instead of X/Y staying at the literal's smallest signed type.
+	testing.expect_value(t, len(diagnostics.items), 0)
+	testing.expect(t, types.equal(hir_module.globals[0].type, types.I32))
+	testing.expect(t, types.equal(hir_module.globals[1].type, types.I32))
+	testing.expect(t, types.equal(hir_module.globals[2].type, types.I32))
+}
+
+@(test)
+contextual_inference_open_constants_adopt_unsigned_demand :: proc(t: ^testing.T) {
+	text := `A :: 10
+B u16 :: A
+P :: 10
+R u32 :: P
+N :: 42
+main :: func() void {}
+`
+	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)
+
+	// An open constant is sign-agnostic until used: it adopts the unsigned family a use
+	// demands (the literal's signed default would block this). Unconstrained N defaults.
+	testing.expect_value(t, len(diagnostics.items), 0)
+	testing.expect(t, types.equal(hir_module.globals[0].type, types.U16)) // A
+	testing.expect(t, types.equal(hir_module.globals[1].type, types.U16)) // B
+	testing.expect(t, types.equal(hir_module.globals[2].type, types.U32)) // P
+	testing.expect(t, types.equal(hir_module.globals[3].type, types.U32)) // R
+	testing.expect(t, types.equal(hir_module.globals[4].type, types.I8))  // N
+}
+
+@(test)
+contextual_inference_rejects_constant_that_does_not_fit_demand :: proc(t: ^testing.T) {
+	text := `BIG :: 100000
+C u8 :: BIG
+main :: func() void {}
+`
+	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)
+
+	// 100000 does not fit u8, so the demand is rejected, BIG defaults to i32, and the
+	// genuine mismatch surfaces at the use's boundary coercion.
+	found := false
+	for diagnostic in diagnostics.items {
+		found = found || strings.contains(diagnostic.message, "cannot implicitly convert i32 to u8")
+	}
+	testing.expect(t, found)
+}
+
+@(test)
+contextual_inference_does_not_cross_call_boundaries :: proc(t: ^testing.T) {
+	text := `echo :: func(p int) int { return p }
+A :: 10
+R u32 :: echo(A)
+main :: func() void {}
+`
+	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)
+
+	// The u32 demand on R must not flow through echo into A (that is L3, deferred). A
+	// stays at its default i8, so the call result fails to coerce to u32.
+	found := false
+	for diagnostic in diagnostics.items {
+		found = found || strings.contains(diagnostic.message, "cannot implicitly convert i8 to u32")
+	}
+	testing.expect(t, found)
+}
+
+@(test)
+contextual_inference_resolves_locals_like_globals :: proc(t: ^testing.T) {
+	text := `take_u16 :: func(v u16) void {}
+get :: func() u16 {
+	c :: 10
+	return c
+}
+main :: func() void {
+	x :: 1000
+	y int :: x
+	z i32 :: y
+	a :: 10
+	b u16 :: a
+	n :: 5
+	take_u16(n)
+	_ = get()
+}
+`
+	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)
+
+	// The same backward propagation works for locals: a constant local adopts the
+	// unsigned/wider type a later use demands (declaration, call argument, or return),
+	// so none of these need an explicit annotation. Without it, i8->u16/u32 would error.
+	testing.expect_value(t, len(diagnostics.items), 0)
+}
+
+@(test)
+contextual_inference_demand_from_function_body_reaches_global :: proc(t: ^testing.T) {
+	text := `take_u16 :: func(v u16) void {}
+G :: 10
+main :: func() void {
+	take_u16(G)
+}
+`
+	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)
+
+	// A demand originating inside a function body (passing G to a u16 parameter) flows
+	// back to the open-constant global G, resolving it to u16.
+	testing.expect_value(t, len(diagnostics.items), 0)
+	testing.expect(t, types.equal(hir_module.globals[0].type, types.U16))
+}
+
+@(test)
+contextual_inference_rejects_local_constant_that_does_not_fit :: proc(t: ^testing.T) {
+	text := `main :: func() void {
+	big :: 100000
+	c u8 :: big
+}
+`
+	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)
+
+	// 100000 does not fit u8, so big keeps its i32 default and the use errors.
+	found := false
+	for diagnostic in diagnostics.items {
+		found = found || strings.contains(diagnostic.message, "cannot implicitly convert i32 to u8")
+	}
+	testing.expect(t, found)
+}