Fixes #3049. Add more tests for unary prefix ++ and -- operators. (#3051)

Add more tests for unary prefix ++ and -- operators.

Author: sgrekhov

Language/Expressions/Unary_Expressions/variable_decrement_t01.dart

@@ -2,8 +2,8 @@
 // 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.
 
-/// @assertion Evaluation of an expression of the form --e is equivalent to
-/// e -= 1.
+/// @assertion An expression of the form `--e` is treated as `(e-=1)`.
+///
 /// @description Checks that evaluation of an expression of the form --e is
 /// equivalent to e -= 1.
 /// @author msyabro

Language/Expressions/Unary_Expressions/variable_decrement_t02.dart

@@ -2,8 +2,8 @@
 // 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.
 
-/// @assertion Evaluation of an expression of the form --e is equivalent to
-/// e -= 1.
+/// @assertion An expression of the form `--e` is treated as `(e-=1)`.
+///
 /// @description Checks that evaluation of an expression of the form --e
 /// results in invoking operator- on the result of e with the proper argument.
 /// @author rodionov

Language/Expressions/Unary_Expressions/variable_decrement_t03.dart

@@ -0,0 +1,59 @@
+// 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.
+
+/// @assertion An expression of the form `--e` is treated as `(e-=1)`.
+///
+/// @description Checks that evaluation of an expression of the form `--e`
+/// results in invoking `operator-` on the result of `e` with the proper
+/// argument. Test nullable types.
+/// @author [email protected]
+/// @issue 56963
+
+import '../../../Utils/expect.dart';
+
+class A {
+  final int value;
+  A(this.value);
+
+  A? operator -(int? other) {
+    if (other != null) {
+      return A(value - other);
+    }
+    return null;
+  }
+}
+
+extension on A? {
+  A? operator -(int other) {
+    if (this != null) {
+      return this! - other;
+    }
+    return null;
+  }
+}
+
+extension on int? {
+  int? operator -(int? other) {
+    if (this != null && other != null) {
+      return this! - other;
+    }
+    return null;
+  }
+}
+
+main() {
+  A? a = 2 > 1 ? A(42) : null;
+  var x = --a ?? A(-1);
+  Expect.equals(41, x.value);
+  a = 2 > 3 ? A(42) : null;
+  x = --a ?? A(-1);
+  Expect.equals(-1, x.value);
+
+  int? i = 2 > 1 ? 42 : null;
+  var y = --i ?? -1;
+  Expect.equals(41, y);
+  i = 2 > 3 ? 42 : null;
+  y = --i ?? -1;
+  Expect.equals(-1, y);
+}

Language/Expressions/Unary_Expressions/variable_increment_t01.dart

@@ -2,8 +2,8 @@
 // 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.
 
-/// @assertion Evaluation of an expression of the form ++e is equivalent to
-/// e += 1.
+/// @assertion An expression of the form `++e` is treated as `(e+=1)`.
+///
 /// @description Checks that evaluation of an expression of the form ++e is
 /// equivalent to e += 1.
 /// @author msyabro

Language/Expressions/Unary_Expressions/variable_increment_t02.dart

@@ -2,8 +2,8 @@
 // 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.
 
-/// @assertion Evaluation of an expression of the form ++e is equivalent to
-/// e += 1.
+/// @assertion An expression of the form `++e` is treated as `(e+=1)`.
+///
 /// @description Checks that evaluation of an expression of the form ++e
 /// results in invoking operator+ on the result of e with the proper argument.
 /// @author rodionov

Language/Expressions/Unary_Expressions/variable_increment_t03.dart

@@ -0,0 +1,59 @@
+// 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.
+
+/// @assertion An expression of the form `++e` is treated as `(e+=1)`.
+///
+/// @description Checks that evaluation of an expression of the form `++e`
+/// results in invoking `operator+` on the result of e with the proper argument.
+/// Test nullable types.
+/// @author [email protected]
+/// @issue 56963
+
+import '../../../Utils/expect.dart';
+
+class A {
+  final int value;
+  A(this.value);
+
+  A? operator +(int? other) {
+    if (other != null) {
+      return A(value + other);
+    }
+    return null;
+  }
+}
+
+extension on A? {
+  A? operator +(int other) {
+    if (this != null) {
+      return this! + other;
+    }
+    return null;
+  }
+}
+
+extension on int? {
+  int? operator +(int? other) {
+    if (this != null && other != null) {
+      return this! + other;
+    }
+    return null;
+  }
+}
+
+main() {
+  A? a = 2 > 1 ? A(1) : null;
+  var x = ++a ?? A(-1);
+  Expect.equals(2, x.value);
+  a = 2 > 3 ? A(1) : null;
+  x = ++a ?? A(-1);
+  Expect.equals(-1, x.value);
+
+  int? i = 2 > 1 ? 42 : null;
+  var y = ++i ?? -1;
+  Expect.equals(43, y);
+  i = 2 > 3 ? 42 : null;
+  y = ++i ?? -1;
+  Expect.equals(-1, y);
+}