broaden type inference from context (assignment statements)

TODO.md

@@ -253,7 +253,7 @@
     ```
 
 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
+  - if statements without a bracketed body must wrap the condition in parentheses UNLESS it's a function call
 
 18. add `defer` statement (inspired by zig)
 

compiler/checker/checker.odin

@@ -1727,7 +1727,7 @@ infer_statements :: proc(
 		case .Assignment:
 			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, local_types)
+				target_type := 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 &&
@@ -1735,10 +1735,16 @@ infer_statements :: proc(
 						_ = merge_infer_local_type(checker, &locals^[local_index], value_type, local_types)
 					}
 				}
+				// Push the target's concrete type backward onto the RHS so a const used
+				// only in an assignment (e.g. `x += speed`) resolves, mirroring how
+				// declarations and returns demand their context (record_demand self-gates
+				// on a concrete demand and only touches fitting open slots).
+				_ = record_demand(checker, statement.expr, target_type, locals^[:], local_types, pkg, file)
 			} 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)
+					_ = record_demand(checker, statement.expr, locals^[local_index].type, locals^[:], local_types, pkg, file)
 				}
 			}
 		case .Expression:

compiler_tests.odin

@@ -6603,6 +6603,33 @@ main :: func() void {
 	testing.expect(t, types.equal(hir_module.globals[0].type, types.U16))
 }
 
+@(test)
+contextual_inference_flows_through_compound_assignment :: proc(t: ^testing.T) {
+	text := `main :: func() void {
+	s :: 5
+	v u16 = 0
+	v += s
+	_ = v
+}
+`
+	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)
+
+	// `s` is used only as the RHS of `v += s`. The assignment target's type (u16) is
+	// demanded backward onto `s`, resolving the open constant; without it the compound
+	// assignment would report "arithmetic requires compatible numeric operands".
+	testing.expect_value(t, len(diagnostics.items), 0)
+}
+
 @(test)
 contextual_inference_rejects_local_constant_that_does_not_fit :: proc(t: ^testing.T) {
 	text := `main :: func() void {