Fix pattern context type schema for cast patterns.

According to the patterns spec, the pattern context type schema for a
cast pattern should be `_`. What was actually implemented was
`Object?`.

This is unlikely to make a difference in practice, since (a) cast
patterns are unlikely to be used in circumstances where the pattern
context type schema makes a difference, and (b) it takes some effort
to come up with expressions whose type inference behavior is differenc
between a schema of `Object?` and a schema of `_`.

Change-Id: I695752c8c163621a34faaa8d62b2b076c8152eb0
Bug: https://github.com/dart-lang/sdk/issues/54640
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/346383
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>

CHANGELOG.md

@@ -1,5 +1,14 @@
 ## 3.4.0
 
+### Language
+
+- **Breaking Change** [#54640][]: The pattern context type schema for
+  cast patterns has been changed from `Object?` to `_` (the unknown
+  type), to align with the specification. This change is not expected
+  to make any difference in practice.
+
+[#54640]: https://github.com/dart-lang/sdk/issues/54640
+
 ### Tools
 
 #### Pub

pkg/_fe_analyzer_shared/lib/src/type_inference/type_analyzer.dart

@@ -387,8 +387,7 @@ mixin TypeAnalyzer<
   /// Computes the type schema for a cast pattern.
   ///
   /// Stack effect: none.
-  TypeSchema analyzeCastPatternSchema() =>
-      operations.typeToSchema(operations.objectQuestionType);
+  TypeSchema analyzeCastPatternSchema() => operations.unknownType;
 
   /// Analyzes a constant pattern.  [node] is the pattern itself, and
   /// [expression] is the constant expression.  Depending on the client's

pkg/_fe_analyzer_shared/test/type_inference/type_inference_test.dart

@@ -1876,13 +1876,9 @@ main() {
       test('Type schema', () {
         var x = Var('x');
         h.run([
-          ifCase(
-            expr('num'),
-            x.pattern().as_('int'),
-            [],
-          ).checkIR('ifCase(expr(num), castPattern(varPattern(x, '
-              'matchedType: int, staticType: int), int, matchedType: num), '
-              'variables(x), true, block(), noop)'),
+          match(x.pattern().as_('int'), expr('num').checkSchema('?')).checkIR(
+              'match(expr(num), castPattern(varPattern(x, '
+              'matchedType: int, staticType: int), int, matchedType: num))'),
         ]);
       });
 

pkg/analyzer/test/src/dart/resolution/cast_pattern_test.dart

@@ -102,7 +102,7 @@ PatternVariableDeclaration
     token: x
     staticElement: self::@function::f::@parameter::x
     staticType: dynamic
-  patternTypeSchema: Object?
+  patternTypeSchema: _
 ''');
   }
 }

tests/language/patterns/schema_test.dart

@@ -62,9 +62,7 @@ test() {
 
   // - Cast: The context type schema is `_`.
   {
-    // TODO(paulberry): uncomment this after
-    // https://github.com/dart-lang/sdk/issues/54640 is addressed.
-    // var [_ as Object] = [1]..expectStaticType<Exactly<List<int>>>();
+    var [_ as Object] = [1]..expectStaticType<Exactly<List<int>>>();
   }
 
   // - Parenthesized: The context type schema of the inner subpattern.