checker: fix checking of default field initialisations, that are part of unions of structs tagged with @[noinit] (#21587)

vlib/v/ast/ast.v

@@ -341,6 +341,8 @@ pub:
 	is_deprecated    bool
 pub mut:
 	is_recursive     bool
+	is_part_of_union bool
+	container_typ    Type
 	default_expr     Expr
 	default_expr_typ Type
 	name             string

vlib/v/checker/struct.v

@@ -14,6 +14,13 @@ fn (mut c Checker) struct_decl(mut node ast.StructDecl) {
 	mut struct_sym, struct_typ_idx := c.table.find_sym_and_type_idx(node.name)
 	mut has_generic_types := false
 	if mut struct_sym.info is ast.Struct {
+		for mut symfield in struct_sym.info.fields {
+			symfield.container_typ = struct_typ_idx
+			if struct_sym.info.is_union {
+				symfield.is_part_of_union = true
+			}
+		}
+
 		if node.language == .v && !c.is_builtin_mod && !struct_sym.info.is_anon {
 			c.check_valid_pascal_case(node.name, 'struct name', node.pos)
 		}
@@ -844,13 +851,23 @@ or use an explicit `unsafe{ a[..] }`, if you do not want a copy of the slice.',
 						node.pos)
 				}
 				if !node.has_update_expr && !field.has_default_expr && !field.typ.is_ptr()
-					&& !field.typ.has_flag(.option) && c.table.final_sym(field.typ).kind == .struct_ {
+					&& !field.typ.has_flag(.option) {
+					field_final_sym := c.table.final_sym(field.typ)
+					if field_final_sym.kind == .struct_ {
 						mut zero_struct_init := ast.StructInit{
 							pos: node.pos
 							typ: field.typ
 						}
+						if field.is_part_of_union {
+							if field.name in inited_fields {
+								// fields that are part of an union, should only be checked, when they are explicitly initialised
 								c.struct_init(mut zero_struct_init, true, mut inited_fields)
 							}
+						} else {
+							c.struct_init(mut zero_struct_init, true, mut inited_fields)
+						}
+					}
+				}
 			}
 			for embed in info.embeds {
 				mut zero_struct_init := ast.StructInit{

vlib/v/tests/modules/structs_with_noinit/shapes.v

@@ -0,0 +1,22 @@
+module structs_with_noinit
+
+@[noinit]
+pub struct Image {}
+
+@[noinit]
+pub struct Rect {}
+
+@[noinit]
+pub struct Circle {}
+
+pub fn make_circle() Circle {
+	return Circle{}
+}
+
+pub fn make_image() Image {
+	return Image{}
+}
+
+pub fn make_rect() Rect {
+	return Rect{}
+}

vlib/v/tests/struct_field_default_union_of_structs_marked_with_noinit_test.v

@@ -0,0 +1,44 @@
+module main
+
+import structs_with_noinit
+
+enum RenderableKind {
+	circle
+	image
+	rect
+}
+
+union RenderableValue {
+	circle structs_with_noinit.Circle
+	image  structs_with_noinit.Image
+	rect   structs_with_noinit.Rect
+}
+
+struct Renderable {
+	RenderableValue
+	kind RenderableKind
+}
+
+fn draw(r Renderable) int {
+	match r.kind {
+		.circle {
+			println('()')
+			return 999
+		}
+		.rect {
+			println('[]')
+		}
+		.image {
+			println('o_O')
+		}
+	}
+	return 1
+}
+
+fn test_initialisation_of_a_struct_containing_embedded_union_field() {
+	r := Renderable{
+		kind: .circle
+		circle: structs_with_noinit.make_circle()
+	}
+	assert draw(r) == 999
+}