checker: fix generic struct init with update expr (fix #17824) (#17827)

vlib/v/checker/struct.v

@@ -295,7 +295,8 @@ fn (mut c Checker) struct_init(mut node ast.StructInit, is_field_zero_struct_ini
 			}
 		}
 		if struct_sym.info.generic_types.len > 0 && struct_sym.info.concrete_types.len == 0
-			&& !node.is_short_syntax && c.table.cur_concrete_types.len != 0 {
+			&& !node.is_short_syntax && c.table.cur_concrete_types.len != 0
+			&& !is_field_zero_struct_init {
 			if node.generic_types.len == 0 {
 				c.error('generic struct init must specify type parameter, e.g. Foo[T]',
 					node.pos)
@@ -648,8 +649,9 @@ fn (mut c Checker) struct_init(mut node ast.StructInit, is_field_zero_struct_ini
 							node.pos)
 					}
 				}
-				if !field.has_default_expr && field.name !in inited_fields && !field.typ.is_ptr()
-					&& !field.typ.has_flag(.option) && c.table.final_sym(field.typ).kind == .struct_ {
+				if !node.has_update_expr && !field.has_default_expr && field.name !in inited_fields
+					&& !field.typ.is_ptr() && !field.typ.has_flag(.option)
+					&& c.table.final_sym(field.typ).kind == .struct_ {
 					mut zero_struct_init := ast.StructInit{
 						pos: node.pos
 						typ: field.typ

vlib/v/tests/generic_struct_init_with_update_expr_test.v

@@ -0,0 +1,39 @@
+pub struct Time[T] {
+pub mut:
+	start T
+	end   T
+}
+
+pub struct Transform[T] {
+pub mut:
+	time   Time[T]
+	before []T
+	after  []T
+}
+
+pub fn (t Transform[T]) clone() Transform[T] {
+	return Transform[T]{
+		...t
+	}
+}
+
+pub fn (t Transform[T]) default() Transform[T] {
+	return Transform[T]{}
+}
+
+fn test_generic_struct_init_with_update_expr() {
+	a := Transform[f64]{
+		before: [0.0, 0.0]
+		after: [320.0, 240.0]
+	}
+
+	b := a.clone()
+	println(b)
+	assert b.before == a.before
+	assert b.after == a.after
+
+	c := a.default()
+	println(c)
+	assert c.before.len == 0
+	assert c.after.len == 0
+}