cgen: cast default struct field value to correct SumType/interface (#8619)

vlib/v/checker/checker.v

@@ -465,9 +465,13 @@ pub fn (mut c Checker) struct_decl(mut decl ast.StructDecl) {
 			if field.has_default_expr {
 				c.expected_type = field.typ
 				field_expr_type := c.expr(field.default_expr)
+				struct_sym.info.fields[i].default_expr_typ = field_expr_type
 				c.check_expected(field_expr_type, field.typ) or {
-					c.error('incompatible initializer for field `$field.name`: $err',
-						field.default_expr.position())
+					if !(sym.kind == .interface_
+						&& c.type_implements(field_expr_type, field.typ, field.pos)) {
+						c.error('incompatible initializer for field `$field.name`: $err',
+							field.default_expr.position())
+					}
 				}
 				// Check for unnecessary inits like ` = 0` and ` = ''`
 				if field.typ.is_ptr() {

vlib/v/gen/c/cgen.v

@@ -5074,8 +5074,14 @@ fn (mut g Gen) struct_init(struct_init ast.StructInit) {
 fn (mut g Gen) zero_struct_field(field table.Field) {
 	field_name := c_name(field.name)
 	g.write('.$field_name = ')
+	sym := g.table.get_type_symbol(field.typ)
+	defex := ast.fe2ex(field.default_expr)
 	if field.has_default_expr {
-		g.expr(ast.fe2ex(field.default_expr))
+		if sym.kind in [.sum_type, .interface_] {
+			g.expr_with_cast(defex, field.default_expr_typ, field.typ)
+			return
+		}
+		g.expr(defex)
 	} else {
 		g.write(g.type_default(field.typ))
 	}

vlib/v/table/types.v

@@ -694,6 +694,7 @@ pub mut:
 	typ              Type
 	default_expr     FExpr
 	has_default_expr bool
+	default_expr_typ Type
 	default_val      string
 	attrs            []Attr
 	is_pub           bool

vlib/v/tests/struct_field_default_value_interface_cast_test.v

@@ -0,0 +1,17 @@
+struct Foo {
+	x int
+}
+
+interface FooBar {
+	x int
+}
+
+struct Abc {
+    foobar FooBar = Foo { x: 123 }
+}
+
+fn test_struct_field_default_value_interface_cast() {
+	x := Abc{}
+	assert x.foobar is Foo
+	assert x.foobar.x == 123
+}

vlib/v/tests/struct_field_default_value_sumtype_cast_test.v

@@ -0,0 +1,17 @@
+struct Foo {
+	x int
+}
+
+struct Bar {}
+
+type FooBar = Foo | Bar
+
+struct Abc {
+    foobar FooBar = Foo { x: 123 }
+}
+
+fn test_struct_field_default_value_sumtype_cast() {
+	x := Abc{}
+	assert x.foobar is Foo
+	assert (x.foobar as Foo).x == 123
+}