[cfe] Avoid crash on Never as bound

Change-Id: Ia7dc56774599a17d4a29d03ec8ec07cb01995fdb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/137300
Reviewed-by: Dmitry Stefantsov <dmitryas@google.com>

pkg/front_end/lib/src/fasta/source/source_library_builder.dart

@@ -3359,7 +3359,8 @@ class SourceLibraryBuilder extends LibraryBuilderImpl {
     }
     if (nnbdIssues != null) {
       if (legacyIssues != null) {
-        nnbdIssues = nnbdIssues.where((issue) => !legacyIssues.contains(issue));
+        nnbdIssues =
+            nnbdIssues.where((issue) => !legacyIssues.contains(issue)).toSet();
       }
       reportTypeArgumentIssues(nnbdIssues, fileUri, offset,
           inferred: inferred, areWarnings: loader.nnbdMode == NnbdMode.Weak);

pkg/front_end/testcases/nnbd/never_bound.dart

@@ -0,0 +1,15 @@
+// Copyright (c) 2020, 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.
+
+class GenericNever<T extends Never> {
+  dynamic getParamType() => T;
+}
+
+errors() {
+  GenericNever<Null>();
+  GenericNever<void>();
+  GenericNever<int>();
+}
+
+main() {}

pkg/front_end/testcases/nnbd/never_bound.dart.outline.expect

@@ -0,0 +1,14 @@
+library /*isNonNullableByDefault*/;
+import self as self;
+import "dart:core" as core;
+
+class GenericNever<T extends Never = Never> extends core::Object {
+  synthetic constructor •() → self::GenericNever<self::GenericNever::T>
+    ;
+  method getParamType() → dynamic
+    ;
+}
+static method errors() → dynamic
+  ;
+static method main() → dynamic
+  ;

pkg/front_end/testcases/nnbd/never_bound.dart.strong.expect

@@ -0,0 +1,44 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/nnbd/never_bound.dart:10:3: Error: Type argument 'Null?' doesn't conform to the bound 'Never' of the type variable 'T' on 'GenericNever'.
+// Try changing type arguments so that they conform to the bounds.
+//   GenericNever<Null>();
+//   ^
+// pkg/front_end/testcases/nnbd/never_bound.dart:5:20: Context: This is the type variable whose bound isn't conformed to.
+// class GenericNever<T extends Never> {
+//                    ^
+//
+// pkg/front_end/testcases/nnbd/never_bound.dart:11:3: Error: Type argument 'void' doesn't conform to the bound 'Never' of the type variable 'T' on 'GenericNever'.
+// Try changing type arguments so that they conform to the bounds.
+//   GenericNever<void>();
+//   ^
+// pkg/front_end/testcases/nnbd/never_bound.dart:5:20: Context: This is the type variable whose bound isn't conformed to.
+// class GenericNever<T extends Never> {
+//                    ^
+//
+// pkg/front_end/testcases/nnbd/never_bound.dart:12:3: Error: Type argument 'int' doesn't conform to the bound 'Never' of the type variable 'T' on 'GenericNever'.
+// Try changing type arguments so that they conform to the bounds.
+//   GenericNever<int>();
+//   ^
+// pkg/front_end/testcases/nnbd/never_bound.dart:5:20: Context: This is the type variable whose bound isn't conformed to.
+// class GenericNever<T extends Never> {
+//                    ^
+//
+import self as self;
+import "dart:core" as core;
+
+class GenericNever<T extends Never = Never> extends core::Object {
+  synthetic constructor •() → self::GenericNever<self::GenericNever::T>
+    : super core::Object::•()
+    ;
+  method getParamType() → dynamic
+    return self::GenericNever::T;
+}
+static method errors() → dynamic {
+  new self::GenericNever::•<core::Null?>();
+  new self::GenericNever::•<void>();
+  new self::GenericNever::•<core::int>();
+}
+static method main() → dynamic {}

pkg/front_end/testcases/nnbd/never_bound.dart.strong.transformed.expect

@@ -0,0 +1,44 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/nnbd/never_bound.dart:10:3: Error: Type argument 'Null?' doesn't conform to the bound 'Never' of the type variable 'T' on 'GenericNever'.
+// Try changing type arguments so that they conform to the bounds.
+//   GenericNever<Null>();
+//   ^
+// pkg/front_end/testcases/nnbd/never_bound.dart:5:20: Context: This is the type variable whose bound isn't conformed to.
+// class GenericNever<T extends Never> {
+//                    ^
+//
+// pkg/front_end/testcases/nnbd/never_bound.dart:11:3: Error: Type argument 'void' doesn't conform to the bound 'Never' of the type variable 'T' on 'GenericNever'.
+// Try changing type arguments so that they conform to the bounds.
+//   GenericNever<void>();
+//   ^
+// pkg/front_end/testcases/nnbd/never_bound.dart:5:20: Context: This is the type variable whose bound isn't conformed to.
+// class GenericNever<T extends Never> {
+//                    ^
+//
+// pkg/front_end/testcases/nnbd/never_bound.dart:12:3: Error: Type argument 'int' doesn't conform to the bound 'Never' of the type variable 'T' on 'GenericNever'.
+// Try changing type arguments so that they conform to the bounds.
+//   GenericNever<int>();
+//   ^
+// pkg/front_end/testcases/nnbd/never_bound.dart:5:20: Context: This is the type variable whose bound isn't conformed to.
+// class GenericNever<T extends Never> {
+//                    ^
+//
+import self as self;
+import "dart:core" as core;
+
+class GenericNever<T extends Never = Never> extends core::Object {
+  synthetic constructor •() → self::GenericNever<self::GenericNever::T>
+    : super core::Object::•()
+    ;
+  method getParamType() → dynamic
+    return self::GenericNever::T;
+}
+static method errors() → dynamic {
+  new self::GenericNever::•<core::Null?>();
+  new self::GenericNever::•<void>();
+  new self::GenericNever::•<core::int>();
+}
+static method main() → dynamic {}

pkg/front_end/testcases/nnbd/never_bound.dart.weak.expect

@@ -0,0 +1,44 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/nnbd/never_bound.dart:10:3: Error: Type argument 'Null?' doesn't conform to the bound 'Never' of the type variable 'T' on 'GenericNever'.
+// Try changing type arguments so that they conform to the bounds.
+//   GenericNever<Null>();
+//   ^
+// pkg/front_end/testcases/nnbd/never_bound.dart:5:20: Context: This is the type variable whose bound isn't conformed to.
+// class GenericNever<T extends Never> {
+//                    ^
+//
+// pkg/front_end/testcases/nnbd/never_bound.dart:11:3: Error: Type argument 'void' doesn't conform to the bound 'Never' of the type variable 'T' on 'GenericNever'.
+// Try changing type arguments so that they conform to the bounds.
+//   GenericNever<void>();
+//   ^
+// pkg/front_end/testcases/nnbd/never_bound.dart:5:20: Context: This is the type variable whose bound isn't conformed to.
+// class GenericNever<T extends Never> {
+//                    ^
+//
+// pkg/front_end/testcases/nnbd/never_bound.dart:12:3: Error: Type argument 'int' doesn't conform to the bound 'Never' of the type variable 'T' on 'GenericNever'.
+// Try changing type arguments so that they conform to the bounds.
+//   GenericNever<int>();
+//   ^
+// pkg/front_end/testcases/nnbd/never_bound.dart:5:20: Context: This is the type variable whose bound isn't conformed to.
+// class GenericNever<T extends Never> {
+//                    ^
+//
+import self as self;
+import "dart:core" as core;
+
+class GenericNever<T extends Never = Never> extends core::Object {
+  synthetic constructor •() → self::GenericNever<self::GenericNever::T>
+    : super core::Object::•()
+    ;
+  method getParamType() → dynamic
+    return self::GenericNever::T;
+}
+static method errors() → dynamic {
+  new self::GenericNever::•<core::Null?>();
+  new self::GenericNever::•<void>();
+  new self::GenericNever::•<core::int>();
+}
+static method main() → dynamic {}

pkg/front_end/testcases/nnbd/never_bound.dart.weak.transformed.expect

@@ -0,0 +1,44 @@
+library /*isNonNullableByDefault*/;
+//
+// Problems in library:
+//
+// pkg/front_end/testcases/nnbd/never_bound.dart:10:3: Error: Type argument 'Null?' doesn't conform to the bound 'Never' of the type variable 'T' on 'GenericNever'.
+// Try changing type arguments so that they conform to the bounds.
+//   GenericNever<Null>();
+//   ^
+// pkg/front_end/testcases/nnbd/never_bound.dart:5:20: Context: This is the type variable whose bound isn't conformed to.
+// class GenericNever<T extends Never> {
+//                    ^
+//
+// pkg/front_end/testcases/nnbd/never_bound.dart:11:3: Error: Type argument 'void' doesn't conform to the bound 'Never' of the type variable 'T' on 'GenericNever'.
+// Try changing type arguments so that they conform to the bounds.
+//   GenericNever<void>();
+//   ^
+// pkg/front_end/testcases/nnbd/never_bound.dart:5:20: Context: This is the type variable whose bound isn't conformed to.
+// class GenericNever<T extends Never> {
+//                    ^
+//
+// pkg/front_end/testcases/nnbd/never_bound.dart:12:3: Error: Type argument 'int' doesn't conform to the bound 'Never' of the type variable 'T' on 'GenericNever'.
+// Try changing type arguments so that they conform to the bounds.
+//   GenericNever<int>();
+//   ^
+// pkg/front_end/testcases/nnbd/never_bound.dart:5:20: Context: This is the type variable whose bound isn't conformed to.
+// class GenericNever<T extends Never> {
+//                    ^
+//
+import self as self;
+import "dart:core" as core;
+
+class GenericNever<T extends Never = Never> extends core::Object {
+  synthetic constructor •() → self::GenericNever<self::GenericNever::T>
+    : super core::Object::•()
+    ;
+  method getParamType() → dynamic
+    return self::GenericNever::T;
+}
+static method errors() → dynamic {
+  new self::GenericNever::•<core::Null?>();
+  new self::GenericNever::•<void>();
+  new self::GenericNever::•<core::int>();
+}
+static method main() → dynamic {}

pkg/front_end/testcases/text_serialization.status

@@ -1268,6 +1268,7 @@ nnbd/missing_required_named_parameter: TextSerializationFailure
 nnbd/mixed_mode_hierarchy_generic_methods: TextSerializationFailure
 nnbd/mixin_from_opt_in: TextSerializationFailure
 nnbd/mixin_from_opt_out: TextSerializationFailure
+nnbd/never_bound: TextSerializationFailure
 nnbd/never_opt_out: TypeCheckError
 nnbd/never_receiver: TextSerializationFailure
 nnbd/nnbd_opt_out_language_version: TextSerializationFailure