Reenable warnings/hints for method type variables in dart2js

Change-Id: I09662afc8b1a4e8c9fdbfe2e342e80ea58351593
Reviewed-on: https://dart-review.googlesource.com/32060
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>

pkg/compiler/lib/src/compile_time_constants.dart

@@ -268,10 +268,6 @@ abstract class ConstantCompilerBase implements ConstantCompiler {
               ErroneousElement element = elementType.element;
               reporter.reportErrorMessage(
                   node, element.messageKind, element.messageArguments);
-            } else {
-              assert(elementType is MethodTypeVariableType);
-              // Used to `reportErrorMessage` here, but with Dart 2 upcoming
-              // very soon we do not emit this message any more.
             }
           } else {
             // We need to throw an exception at runtime.

pkg/compiler/lib/src/diagnostics/messages.dart

@@ -404,6 +404,8 @@ enum MessageKind {
   TYPE_ARGUMENT_COUNT_MISMATCH,
   TYPE_VARIABLE_IN_CONSTANT,
   TYPE_VARIABLE_WITHIN_STATIC_MEMBER,
+  TYPE_VARIABLE_FROM_METHOD_NOT_REIFIED,
+  TYPE_VARIABLE_FROM_METHOD_CONSIDERED_DYNAMIC,
   TYPEDEF_FORMAL_WITH_DEFAULT,
   UNARY_OPERATOR_BAD_ARITY,
   UNBOUND_LABEL,
@@ -1195,6 +1197,43 @@ class C<T> {
 }
 
 void main() => new C().m(null);
+"""
+          ]),
+
+      MessageKind.TYPE_VARIABLE_FROM_METHOD_NOT_REIFIED: const MessageTemplate(
+          MessageKind.TYPE_VARIABLE_FROM_METHOD_NOT_REIFIED,
+          "Method type variables do not have a runtime value.",
+          howToFix: "Try using the upper bound of the type variable, "
+              "or refactor the code to avoid needing this runtime value.",
+          examples: const [
+            """
+// Method type variables are not reified, so they cannot be returned.
+Type f<T>() => T;
+
+main() => f<int>();
+""",
+            """
+// Method type variables are not reified, so they cannot be tested dynamically.
+bool f<T>(Object o) => o is T;
+
+main() => f<int>(42);
+"""
+          ]),
+
+      MessageKind.TYPE_VARIABLE_FROM_METHOD_CONSIDERED_DYNAMIC:
+          const MessageTemplate(
+              MessageKind.TYPE_VARIABLE_FROM_METHOD_CONSIDERED_DYNAMIC,
+              "Method type variables are treated as `dynamic` in `as` "
+              "expressions.",
+              howToFix:
+                  "Try using the upper bound of the type variable, or check "
+                  "that the blind success of the test does not introduce bugs.",
+              examples: const [
+            """
+// Method type variables are not reified, so they cannot be tested dynamically.
+bool f<T>(Object o) => o as T;
+
+main() => f<int>(42);
 """
           ]),
 

pkg/compiler/lib/src/resolution/members.dart

@@ -1131,6 +1131,13 @@ class ResolverVisitor extends MappingVisitor<ResolutionResult> {
       sendStructure = new IsStructure(type);
     }
 
+    // GENERIC_METHODS: Method type variables are not reified so we must warn
+    // about the error which will occur at runtime.
+    if (type is MethodTypeVariableType) {
+      reporter.reportWarningMessage(
+          node, MessageKind.TYPE_VARIABLE_FROM_METHOD_NOT_REIFIED);
+    }
+
     registry.registerTypeUse(new TypeUse.isCheck(type));
     registry.registerSendStructure(node, sendStructure);
     return const NoneResult();
@@ -1145,6 +1152,13 @@ class ResolverVisitor extends MappingVisitor<ResolutionResult> {
     ResolutionDartType type =
         resolveTypeAnnotation(typeNode, registerCheckedModeCheck: false);
 
+    // GENERIC_METHODS: Method type variables are not reified, so we must inform
+    // the developer about the potentially bug-inducing semantics.
+    if (type is MethodTypeVariableType) {
+      reporter.reportHintMessage(
+          node, MessageKind.TYPE_VARIABLE_FROM_METHOD_CONSIDERED_DYNAMIC);
+    }
+
     registry.registerTypeUse(new TypeUse.asCast(type));
     registry.registerSendStructure(node, new AsStructure(type));
     return const NoneResult();
@@ -1913,6 +1927,12 @@ class ResolverVisitor extends MappingVisitor<ResolutionResult> {
       // TODO(johnniwinther): Clean up registration of elements and selectors
       // for this case.
     } else {
+      // GENERIC_METHODS: Method type variables are not reified so we must warn
+      // about the error which will occur at runtime.
+      if (element.type is MethodTypeVariableType) {
+        reporter.reportWarningMessage(
+            node, MessageKind.TYPE_VARIABLE_FROM_METHOD_NOT_REIFIED);
+      }
       semantics = new StaticAccess.typeParameterTypeLiteral(element);
     }
 

tests/compiler/dart2js/old_frontend/method_type_variable_test.dart

@@ -0,0 +1,141 @@
+import 'package:async_helper/async_helper.dart';
+import 'package:compiler/src/diagnostics/messages.dart';
+import 'package:expect/expect.dart';
+import '../memory_compiler.dart';
+
+runTest(String code,
+    {List<MessageKind> expectedWarnings: const <MessageKind>[],
+    List<MessageKind> expectedHints: const <MessageKind>[]}) async {
+  print('--test--------------------------------------------------------------');
+  print(code);
+  DiagnosticCollector collector = new DiagnosticCollector();
+  await runCompiler(
+      memorySourceFiles: {'main.dart': code}, diagnosticHandler: collector);
+  Expect.equals(0, collector.errors.length, "Unexpected errors.");
+  Expect.listEquals(
+      expectedWarnings,
+      collector.warnings.map((m) => m.messageKind).toList(),
+      "Unexpected warnings.");
+  Expect.listEquals(expectedHints,
+      collector.hints.map((m) => m.messageKind).toList(), "Unexpected hints.");
+}
+
+class Test {
+  final String code;
+  final List<MessageKind> warnings;
+  final List<MessageKind> hints;
+
+  const Test(this.code,
+      {this.warnings: const <MessageKind>[],
+      this.hints: const <MessageKind>[]});
+}
+
+const List<Test> tests = const <Test>[
+  /// Is-test on method type variable in unused static method.
+  const Test('''
+method<T>(T t) => t is T;
+main() {}
+'''),
+
+  /// Is-test on method type variable in used static method.
+  const Test('''
+method<T>(T t) => t is T;
+main() => method<int>(0);
+''', warnings: const <MessageKind>[
+    MessageKind.TYPE_VARIABLE_FROM_METHOD_NOT_REIFIED
+  ]),
+
+  /// Is-test on method type variable in unused instance method.
+  const Test('''
+class C {
+  method<T>(T t) => t is T;
+}
+main() => new C();
+'''),
+
+  /// Is-test on method type variable in used instance method.
+  const Test('''
+class C {
+  method<T>(T t) => t is T;
+}
+main() => new C().method<int>(0);
+''', warnings: const <MessageKind>[
+    MessageKind.TYPE_VARIABLE_FROM_METHOD_NOT_REIFIED
+  ]),
+
+  /// As-cast on method type variable in unused static method.
+  const Test('''
+method<T>(T t) => t as T;
+main() {}
+'''),
+
+  /// As-cast on method type variable in used static method.
+  const Test('''
+method<T>(T t) => t as T;
+main() => method<int>(0);
+''', hints: const <MessageKind>[
+    MessageKind.TYPE_VARIABLE_FROM_METHOD_CONSIDERED_DYNAMIC
+  ]),
+
+  /// As-cast on method type variable in unused instance method.
+  const Test('''
+class C {
+  method<T>(T t) => t as T;
+}
+main() => new C();
+'''),
+
+  /// As-cast on method type variable in used instance method.
+  const Test('''
+class C {
+  method<T>(T t) => t as T;
+}
+main() => new C().method<int>(0);
+''', hints: const <MessageKind>[
+    MessageKind.TYPE_VARIABLE_FROM_METHOD_CONSIDERED_DYNAMIC
+  ]),
+
+  /// Method type variable literal in unused static method.
+  const Test('''
+method<T>() => T;
+main() {}
+'''),
+
+  /// Method type variable literal in used static method.
+  const Test('''
+method<T>() => T;
+main() => method<int>();
+''', warnings: const <MessageKind>[
+    MessageKind.TYPE_VARIABLE_FROM_METHOD_NOT_REIFIED
+  ]),
+
+  /// Method type variable literal in unused instance method.
+  const Test('''
+class C {
+  method<T>() => T;
+}
+main() => new C();
+'''),
+
+  /// Method type variable literal in used instance method.
+  const Test('''
+class C {
+  method<T>() => T;
+}
+main() => new C().method<int>();
+''', warnings: const <MessageKind>[
+    MessageKind.TYPE_VARIABLE_FROM_METHOD_NOT_REIFIED
+  ]),
+];
+
+main() {
+  asyncTest(() async {
+    for (Test test in tests) {
+      await runTest(
+        test.code,
+        expectedWarnings: test.warnings,
+        expectedHints: test.hints,
+      );
+    }
+  });
+}