expand yield to if-statements and loops

compiler_tests.odin

@@ -2131,8 +2131,10 @@ yield_compiles_and_runs :: proc(t: ^testing.T) {
 	status := compiler_core.compile_package("examples/programs/yield", output)
 	testing.expect_value(t, status, 0)
 	state := run_executable(output)
-	// Untyped/typed value blocks, the value captured before block defers run, and
-	// reassignment from a value block together produce 42.
+	// Value blocks (untyped/typed, defer-spill, reassignment) plus value if-statements
+	// (untyped/typed/reassign/else-if/defer) and value loops (a labeled `for` search
+	// yielding `?usize` on both the found and not-found paths, and a labeled `while`)
+	// together produce 42.
 	testing.expect_value(t, state.exit_code, 42)
 }
 
@@ -2175,6 +2177,57 @@ yield_misuse_is_diagnosed :: proc(t: ^testing.T) {
 	testing.expect(t, misplaced_yield)
 }
 
+@(test)
+yield_control_flow_is_diagnosed :: proc(t: ^testing.T) {
+	// Value if/loop misuse (milestone 20.5): an `if` value without an `else`; a
+	// branch that does not end in `yield`; a value loop whose body lacks a trailing
+	// fall-through `yield`; and a `yield :label` with no matching value loop.
+	text := `main :: func() i32 {
+	noelse :: if (true) {
+		yield 1
+	}
+	badbranch :: if (true) {
+		k :: 5
+	} else {
+		yield 2
+	}
+	noloopyield :: for 0..10 |i| blk: {
+		if (i == 0) yield :blk i
+	}
+	for 0..10 |j| stray: {
+		yield :stray j
+	}
+	return noelse + badbranch + noloopyield
+}
+`
+	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)
+
+	no_else := false
+	branch_no_yield := false
+	loop_no_yield := false
+	stray_label := false
+	for diagnostic in diagnostics.items {
+		no_else = no_else || strings.contains(diagnostic.message, "an 'if' used as a value must have an 'else'")
+		branch_no_yield = branch_no_yield || strings.contains(diagnostic.message, "a value block must end with an explicit 'yield'")
+		loop_no_yield = loop_no_yield || strings.contains(diagnostic.message, "a value loop's body must end with a 'yield'")
+		stray_label = stray_label || strings.contains(diagnostic.message, "no enclosing value loop is labeled 'stray'")
+	}
+	testing.expect(t, no_else)
+	testing.expect(t, branch_no_yield)
+	testing.expect(t, loop_no_yield)
+	testing.expect(t, stray_label)
+}
+
 @(test)
 foreign_function_links_from_c_source :: proc(t: ^testing.T) {
 	output := "/tmp/brolang-test-foreign-source"