Check result type of awaiter

docs/features/async-streams.md

@@ -101,7 +101,7 @@ finally
 ### Detailed design for async-iterator methods
 
 An async-iterator method is replaced by a kick-off method, which initializes a state machine. It does not start running the state machine (unlike kick-off methods for regular async method).
-The kick-off method method is marked with both `AsyncStateMachineAttribute` and `IteratorStateMachineAttribute`.
+The kick-off method method is marked with `AsyncIteratorStateMachineAttribute`.
 
 The state machine for an enumerable async-iterator method primarily implements `IAsyncEnumerable<T>` and `IAsyncEnumerator<T>`.
 For an enumerator async-iterator, it only implements `IAsyncEnumerator<T>`.

src/Compilers/CSharp/Portable/Binder/ForEachLoopBinder.cs

@@ -210,6 +210,12 @@ private BoundForEachStatement BindForEachPartsWorker(DiagnosticBag diagnostics,
             {
                 BoundExpression placeholder = new BoundAwaitableValuePlaceholder(_syntax.Expression, builder.MoveNextMethod?.ReturnType.TypeSymbol ?? CreateErrorType());
                 awaitInfo = BindAwaitInfo(placeholder, _syntax.Expression, _syntax.AwaitKeyword.GetLocation(), diagnostics, ref hasErrors);
+
+                if (!hasErrors && awaitInfo.GetResult?.ReturnType.SpecialType != SpecialType.System_Boolean)
+                {
+                    diagnostics.Add(ErrorCode.ERR_BadGetAsyncEnumerator, _syntax.Expression.Location, collectionExpr.Type, GetAsyncEnumeratorMethodName);
+                    hasErrors = true;
+                }
             }
 
             TypeSymbol iterationVariableType;

src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAwaitForeachTests.cs

@@ -567,11 +567,10 @@ public int Current
     }
 }";
             var comp = CreateCompilationWithMscorlib46(source);
-            // TODO
             comp.VerifyDiagnostics(
-                // (6,33): error CS8412: Asynchronous foreach requires that the return type 'C.Enumerator' of 'C.GetAsyncEnumerator(CancellationToken)' must have a suitable public 'MoveNextAsync' method and public 'Current' property
+                // (6,33): error CS8412: Asynchronous foreach requires that the return type 'C' of 'GetAsyncEnumerator' must have a suitable public 'MoveNextAsync' method and public 'Current' property
                 //         await foreach (var i in new C())
-                Diagnostic(ErrorCode.ERR_BadGetAsyncEnumerator, "new C()").WithArguments("C.Enumerator", "C.GetAsyncEnumerator(System.Threading.CancellationToken)").WithLocation(6, 33)
+                Diagnostic(ErrorCode.ERR_BadGetAsyncEnumerator, "new C()").WithArguments("C", "GetAsyncEnumerator").WithLocation(6, 33)
                 );
         }
 
@@ -604,11 +603,10 @@ public int Current
     }
 }";
             var comp = CreateCompilationWithMscorlib46(source);
-            // TODO
             comp.VerifyDiagnostics(
-                // (6,33): error CS8412: Asynchronous foreach requires that the return type 'C.Enumerator' of 'C.GetAsyncEnumerator(CancellationToken)' must have a suitable public 'MoveNextAsync' method and public 'Current' property
+                // (6,33): error CS8412: Asynchronous foreach requires that the return type 'C' of 'GetAsyncEnumerator' must have a suitable public 'MoveNextAsync' method and public 'Current' property
                 //         await foreach (var i in new C())
-                Diagnostic(ErrorCode.ERR_BadGetAsyncEnumerator, "new C()").WithArguments("C.Enumerator", "C.GetAsyncEnumerator(System.Threading.CancellationToken)").WithLocation(6, 33)
+                Diagnostic(ErrorCode.ERR_BadGetAsyncEnumerator, "new C()").WithArguments("C", "GetAsyncEnumerator").WithLocation(6, 33)
                 );
         }
 
@@ -1983,7 +1981,7 @@ public async Task<bool> MoveNextAsync()
             i = i + 100; // Note: side-effects of async methods in structs are lost
             return more;
         }
-        public async ValueTask DisposeAsync() => throw null;
+        public ValueTask DisposeAsync() => throw null;
     }
 }";
             var comp = CreateCompilationWithTasksExtensions(source + s_IAsyncEnumerable, options: TestOptions.DebugExe);
@@ -2069,7 +2067,8 @@ static async Task Main()
     {
         await foreach (var i in new C())
         {
-            Write(""SKIPPED"");
+            Write($""Item({i}) "");
+            break;
         }
         Write($""Done"");
     }
@@ -2079,7 +2078,7 @@ public AsyncEnumerator GetAsyncEnumerator(System.Threading.CancellationToken tok
     }
     public class AsyncEnumerator : System.IAsyncDisposable
     {
-        public int Current => throw null;
+        public int Current => 1;
         public Awaitable MoveNextAsync()
         {
             return new Awaitable();
@@ -2098,14 +2097,13 @@ public class Awaitable
     public class Awaiter : System.Runtime.CompilerServices.INotifyCompletion
     {
         public bool IsCompleted { get { return true; } }
-        public bool GetResult() { Write(""GetResult ""); return false; }
+        public bool GetResult() { return true; }
         public void OnCompleted(System.Action continuation) { }
     }
 }";
             var comp = CreateCompilationWithTasksExtensions(new[] { source, s_IAsyncEnumerable }, options: TestOptions.DebugExe);
             comp.VerifyDiagnostics();
-            // TODO
-            var v = CompileAndVerify(comp, expectedOutput: "GetResult Dispose Done");
+            var v = CompileAndVerify(comp, expectedOutput: "Item(1) Dispose Done");
 
             var tree = comp.SyntaxTrees.First();
             var model = (SyntaxTreeSemanticModel)comp.GetSemanticModel(tree, ignoreAccessibility: false);

src/Compilers/CSharp/Test/IOperation/IOperation/IOperationTests_IForEachLoopStatement.cs

@@ -3807,14 +3807,14 @@ static async System.Threading.Tasks.Task Main(System.Collections.Generic.IAsyncE
 ";
             // https://github.com/dotnet/roslyn/issues/30362 how do we flag `await`?
             string expectedOperationTree = @"
-IForEachLoopOperation (LoopKind.ForEach, Continue Label Id: 0, Exit Label Id: 1) (OperationKind.Loop, Type: null, IsInvalid) (Syntax: 'await forea ... }')
+IForEachLoopOperation (LoopKind.ForEach, Continue Label Id: 0, Exit Label Id: 1) (OperationKind.Loop, Type: null) (Syntax: 'await forea ... }')
   Locals: Local_1: System.String value
   LoopControlVariable: 
     IVariableDeclaratorOperation (Symbol: System.String value) (OperationKind.VariableDeclarator, Type: null) (Syntax: 'string')
       Initializer: 
         null
   Collection: 
-    IParameterReferenceOperation: pets (OperationKind.ParameterReference, Type: System.Collections.Generic.IAsyncEnumerable<System.String>, IsInvalid) (Syntax: 'pets')
+    IParameterReferenceOperation: pets (OperationKind.ParameterReference, Type: System.Collections.Generic.IAsyncEnumerable<System.String>) (Syntax: 'pets')
   Body: 
     IBlockOperation (1 statements) (OperationKind.Block, Type: null) (Syntax: '{ ... }')
       IExpressionStatementOperation (OperationKind.ExpressionStatement, Type: null) (Syntax: 'System.Cons ... ine(value);')