[tests] Add basic declaring constructors tests.

Adds very basic declaring constructors tests. These are syntax tests and
tests on allowing generative constructors together with a primary constructor only if they are redirecting.

Bug: #61687
Change-Id: Ie8772b9f5cf5705e6f9cad29e6d6b99f8d012d98
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/454161
Reviewed-by: Erik Ernst <[email protected]>
Commit-Queue: Kallen Tu <[email protected]>

Author: kallentu

tests/language/declaring_constructors/body/body_syntax_test.dart

@@ -0,0 +1,30 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// A basic declaring body constructor.
+
+// SharedOptions=--enable-experiment=declaring-constructors
+
+import 'package:expect/expect.dart';
+
+class Point {
+  this(var int x, var int y);
+}
+
+class PointFinal {
+  this(final int x, final int y);
+}
+
+void main() {
+  var p1 = Point(1, 2);
+  Expect.equals(1, p1.x);
+  Expect.equals(2, p1.y);
+
+  p1.x = 3;
+  Expect.equals(3, p1.x);
+
+  var p2 = PointFinal(3, 4);
+  Expect.equals(3, p2.x);
+  Expect.equals(4, p2.y);
+}

tests/language/declaring_constructors/feature_disabled_error_test.dart

@@ -0,0 +1,12 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Declaring constructors are not enabled in versions before release.
+
+// @dart=3.9
+
+class Point(var int x, var int y);
+//         ^
+// [analyzer] unspecified
+// [cfe] unspecified

tests/language/declaring_constructors/feature_enabled_error_test.dart

@@ -0,0 +1,10 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Declaring constructors are not enabled by default.
+
+class Point(var int x, var int y);
+//         ^
+// [analyzer] unspecified
+// [cfe] unspecified

tests/language/declaring_constructors/header/header_generative_constructor_error_test.dart

@@ -0,0 +1,63 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// A class that has a declaring header constructor cannot have any other
+// non-redirecting generative constructors.
+
+// SharedOptions=--enable-experiment=declaring-constructors
+
+class C1(var int x) {
+  C1.named(this.x);
+  // ^
+  // [analyzer] unspecified
+  // [cfe] unspecified
+}
+
+class C2(final int x) {
+  C2.named(this.x);
+  // ^
+  // [analyzer] unspecified
+  // [cfe] unspecified
+}
+
+class C3(int x) {
+  C3.named(int x);
+  // ^
+  // [analyzer] unspecified
+  // [cfe] unspecified
+}
+class C4() {
+  C4.named(int x);
+  // ^
+  // [analyzer] unspecified
+  // [cfe] unspecified
+}
+
+class C5.named(var int x) {
+  C5(this.x);
+  // ^
+  // [analyzer] unspecified
+  // [cfe] unspecified
+}
+
+class C6.named(final int x) {
+  C6(this.x);
+  // ^
+  // [analyzer] unspecified
+  // [cfe] unspecified
+}
+
+class C7.named(int x) {
+  C7(int x);
+  // ^
+  // [analyzer] unspecified
+  // [cfe] unspecified
+}
+
+class C8.named() {
+  C8(int x);
+  // ^
+  // [analyzer] unspecified
+  // [cfe] unspecified
+}

tests/language/declaring_constructors/header/header_generative_constructor_test.dart

@@ -0,0 +1,28 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// A class that has a declaring header constructor can have redirecting
+// generative constructors and factory constructors.
+
+// SharedOptions=--enable-experiment=declaring-constructors
+
+import 'package:expect/expect.dart';
+
+class C1(final int x) {
+  C1.redirecting(int x): this(x);
+}
+
+class C2.named(final int x) {
+  C2.redirecting(int x): this.named(x);
+}
+
+class C3(final int x) {
+  factory C3.from(int value) => C3(value * 2);
+}
+
+void main() {
+  Expect.equals(1, C1.redirecting(1).x);
+  Expect.equals(2, C2.redirecting(2).x);
+  Expect.equals(6, C3.from(3).x);
+}

tests/language/declaring_constructors/header/header_syntax_test.dart

@@ -0,0 +1,26 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// A basic declaring header constructor.
+
+// SharedOptions=--enable-experiment=declaring-constructors
+
+import 'package:expect/expect.dart';
+
+class Point(var int x, var int y);
+
+class PointFinal(final int x, final int y);
+
+void main() {
+  var p1 = Point(1, 2);
+  Expect.equals(1, p1.x);
+  Expect.equals(2, p1.y);
+
+  p1.x = 3;
+  Expect.equals(3, p1.x);
+
+  var p2 = PointFinal(3, 4);
+  Expect.equals(3, p2.x);
+  Expect.equals(4, p2.y);
+}