Language test of record demotion behavior.

When a local variable is promoted to a record type, and then an
assignment statement is used to assign a record literal to that local
variable, if the fields of the new record literal are not assignable
to the fields of the promoted record type, that's not a problem; both
the analyzer and front end agree that the local variable is simply
demoted.

But the spec implies that if the old and new record _shapes_ are the
same, then a compile-time error will occur instead of a demotion. I've
created dart-lang/language#3613 to bring the
spec in line with the implementations. This test demonstrates the
current behavior of the implementations, and makes sure that it
doesn't regress.

Change-Id: I0eacd7ca7f6579a35dbc34687113a2112418f368
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/352462
Reviewed-by: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

Author: stereotype441

tests/language/records/assign_to_promoted_record_test.dart

@@ -0,0 +1,72 @@
+// Copyright (c) 2024, 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.
+
+/// Test that if an assignment is made to a local variable that's been promoted
+/// to a record type, and the newly assigned value doesn't match the previously
+/// promoted type, no error occurs (the variable is simply demoted).
+
+import '../static_type_helper.dart';
+
+void testPositionalFieldTypeMismatch(Object o) {
+  if (o is (int,)) {
+    o.expectStaticType<Exactly<(int,)>>();
+    // Note: prior to https://github.com/dart-lang/language/pull/3613 this would
+    // have been a compile-time error, since there is no coercion from `String`
+    // to `int`.
+    o = ('',)..expectStaticType<Exactly<(String,)>>();
+    o.expectStaticType<Exactly<Object>>();
+  }
+}
+
+void testNamedFieldTypeMismatch(Object o) {
+  if (o is ({int f1})) {
+    o.expectStaticType<Exactly<({int f1})>>();
+    // Note: prior to https://github.com/dart-lang/language/pull/3613 this would
+    // have been a compile-time error, since there is no coercion from `String`
+    // to `int`.
+    o = (f1: '')..expectStaticType<Exactly<({String f1})>>();
+    o.expectStaticType<Exactly<Object>>();
+  }
+}
+
+void testAdditionalPositionalField(Object o) {
+  if (o is (int,)) {
+    o.expectStaticType<Exactly<(int,)>>();
+    o = (1, 2)..expectStaticType<Exactly<(int, int)>>();
+    o.expectStaticType<Exactly<Object>>();
+  }
+}
+
+void testAdditionalNamedField(Object o) {
+  if (o is ({int f1})) {
+    o.expectStaticType<Exactly<({int f1})>>();
+    o = (f1: 1, f2: 2)..expectStaticType<Exactly<({int f1, int f2})>>();
+    o.expectStaticType<Exactly<Object>>();
+  }
+}
+
+void testMissingPositionalField(Object o) {
+  if (o is (int, int)) {
+    o.expectStaticType<Exactly<(int, int)>>();
+    o = (1,);
+    o.expectStaticType<Exactly<Object>>();
+  }
+}
+
+void testMissingNamedField(Object o) {
+  if (o is ({int f1, int f2})) {
+    o.expectStaticType<Exactly<({int f1, int f2})>>();
+    o = (f1: 1);
+    o.expectStaticType<Exactly<Object>>();
+  }
+}
+
+main() {
+  testPositionalFieldTypeMismatch((1,));
+  testNamedFieldTypeMismatch((f1: 1));
+  testAdditionalPositionalField((1,));
+  testAdditionalNamedField((f1: 1));
+  testMissingPositionalField((1, 2));
+  testMissingNamedField((f1: 1, f2: 2));
+}