checker: fix checking return type call disregarding unwrapping (fix #25140) (#25143)

vlib/v/checker/struct.v

@@ -851,7 +851,10 @@ or use an explicit `unsafe{ a[..] }`, if you do not want a copy of the slice.',
 				if mut init_field.expr is ast.CallExpr
 					&& init_field.expr.return_type.has_flag(.generic) {
 					expected_type := c.unwrap_generic(init_field.expected_type)
-					got_type_ret := c.unwrap_generic(init_field.expr.return_type)
+					mut got_type_ret := c.unwrap_generic(init_field.expr.return_type)
+					if init_field.expr.or_block.kind != .absent {
+						got_type_ret = got_type_ret.clear_option_and_result()
+					}
 					if expected_type != got_type_ret {
 						c.error('cannot assign `${c.table.type_to_str(got_type_ret)}` to struct field `${init_field.name}` with type `${c.table.type_to_str(expected_type)}`',
 							init_field.expr.pos)

vlib/v/tests/generics/generic_struct_field_unwrap_test.v

@@ -0,0 +1,23 @@
+struct Test[T] {
+pub:
+	value T
+}
+
+struct AuxTest[T] {
+pub:
+	any_value Test[T]
+}
+
+fn decode[T](arg Test[T]) !Test[T] {
+	return Test[T]{}
+}
+
+pub fn initializing[T]() !AuxTest[T] {
+	return AuxTest[T]{
+		any_value: decode[T](Test[T]{})!
+	}
+}
+
+fn test_main() {
+	dump(initializing[int]()!)
+}