Support `void` as generic argument.

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

 // Copyright (c) 2011, 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.
 
 /**
  * The messages in this file should meet the following guide lines:
  *
  * 1. The message should be a complete sentence starting with an uppercase
  * letter, and ending with a period.
  *
@@ skipping 74 matching lines @@
   ASSERT_IS_GIVEN_NAMED_ARGUMENTS,
   ASSIGNING_FINAL_FIELD_IN_SUPER,
   ASSIGNING_METHOD,
   ASSIGNING_METHOD_IN_SUPER,
   ASSIGNING_TYPE,
   ASYNC_AWAIT_NOT_SUPPORTED,
   ASYNC_KEYWORD_AS_IDENTIFIER,
   ASYNC_MODIFIER_ON_ABSTRACT_METHOD,
   ASYNC_MODIFIER_ON_CONSTRUCTOR,
   ASYNC_MODIFIER_ON_SETTER,
+  AWAIT_FORIN_VOID_GENERIC,
   AWAIT_MEMBER_NOT_FOUND,
   AWAIT_MEMBER_NOT_FOUND_IN_CLOSURE,
   BAD_INPUT_CHARACTER,
   BEFORE_TOP_LEVEL,
   BINARY_OPERATOR_BAD_ARITY,
   BODY_EXPECTED,
   CANNOT_EXTEND,
   CANNOT_EXTEND_ENUM,
   CANNOT_EXTEND_MALFORMED,
   CANNOT_FIND_CONSTRUCTOR,
@@ skipping 89 matching lines @@
   EXPORT_BEFORE_PARTS,
   EXTERNAL_WITH_BODY,
   EXTRA_CATCH_DECLARATION,
   EXTRA_FORMALS,
   EXTRANEOUS_MODIFIER,
   EXTRANEOUS_MODIFIER_REPLACE,
   FACTORY_REDIRECTION_IN_NON_FACTORY,
   FINAL_FUNCTION_TYPE_PARAMETER,
   FINAL_WITHOUT_INITIALIZER,
   FORIN_NOT_ASSIGNABLE,
+  FORIN_VOID_GENERIC,
   FORMAL_DECLARED_CONST,
   FORMAL_DECLARED_STATIC,
   FUNCTION_TYPE_FORMAL_WITH_DEFAULT,
   FUNCTION_WITH_INITIALIZER,
   GENERIC,
   GETTER_MISMATCH,
   UNDEFINED_INSTANCE_GETTER_BUT_SETTER,
   HEX_DIGIT_EXPECTED,
   HIDDEN_HINTS,
   HIDDEN_IMPLICIT_IMPORT,
@@ skipping 300 matching lines @@
       /// 4 of the guide lines for error messages in the beginning of the file.
       MessageKind.GENERIC:
           const MessageTemplate(MessageKind.GENERIC, '#{text}'),
 
       MessageKind.VOID_EXPRESSION: const MessageTemplate(
           MessageKind.VOID_EXPRESSION, "Expression does not yield a value."),
 
       MessageKind.VOID_VARIABLE: const MessageTemplate(
           MessageKind.VOID_VARIABLE, "Variable cannot be of type void."),
 
+      MessageKind.FORIN_VOID_GENERIC: const MessageTemplate(
+          MessageKind.FORIN_VOID_GENERIC,
+          "Generic type of Iterable cannot be of type void.",
+          howToFix: "Try casting the iterable.",

[Siggi Cherem (dart-lang), 2017/03/13 22:50:58]

can a cast succeed? Doesn't this error imply that there is no way to read the
elements of the collection?

[floitsch, 2017/03/14 13:30:49]

Removed the "howToFix". There doesn't seem to be a good suggestion.

+          examples: const [
+            """
+main() {
+  for (var x in new List<void>()) {
+  }
+}
+"""
+            ]),
+
       MessageKind.RETURN_VALUE_IN_VOID: const MessageTemplate(
           MessageKind.RETURN_VALUE_IN_VOID,
           "Cannot return value from void function."),
 
       MessageKind.RETURN_NOTHING: const MessageTemplate(
           MessageKind.RETURN_NOTHING,
           "Value of type '#{returnType}' expected."),
 
       MessageKind.MISSING_ARGUMENT: const MessageTemplate(
           MessageKind.MISSING_ARGUMENT,
           "Missing argument of type '#{argumentType}'."),
 
       MessageKind.ADDITIONAL_ARGUMENT: const MessageTemplate(
           MessageKind.ADDITIONAL_ARGUMENT, "Additional argument."),
 
       MessageKind.NAMED_ARGUMENT_NOT_FOUND: const MessageTemplate(
           MessageKind.NAMED_ARGUMENT_NOT_FOUND,
           "No named argument '#{argumentName}' found on method."),
 
+      MessageKind.AWAIT_FORIN_VOID_GENERIC: const MessageTemplate(
+          MessageKind.AWAIT_FORIN_VOID_GENERIC,
+          "Generic type of Stream cannot be of type void.",
+          howToFix: "Try casting the stream.",

[Siggi Cherem (dart-lang), 2017/03/13 22:50:58]

same

[floitsch, 2017/03/14 13:30:49]

ditto.

+          examples: const [
+            """
+import 'dart:async';
+main() async {
+  await for (var x in new StreamView<void>(null)) {
+  }
+}
+"""
+          ]),
+
       MessageKind.AWAIT_MEMBER_NOT_FOUND: const MessageTemplate(
           MessageKind.AWAIT_MEMBER_NOT_FOUND,
           "No member named 'await' in class '#{className}'.",
           howToFix: "Did you mean to add the 'async' marker "
               "to '#{functionName}'?",
           examples: const [
             """
 class A {
   m() => await -3;
 }
@@ skipping 3265 matching lines @@
   static String convertToString(value) {
     if (value is ErrorToken) {
       // Shouldn't happen.
       return value.assertionMessage;
     } else if (value is Token) {
       value = value.value;
     }
     return '$value';
   }
 }