broaden type inference from context (assignment statements)

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 {