Async-streams: Fix crash with async-iterator method with only throw (#31580)

src/Compilers/CSharp/Portable/CSharpResources.Designer.cs

@@ -8178,6 +8178,24 @@ internal class CSharpResources {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to The body of an async-iterator method must contain a &apos;yield&apos; statement..
+        /// </summary>
+        internal static string ERR_PossibleAsyncIteratorWithoutYield {
+            get {
+                return ResourceManager.GetString("ERR_PossibleAsyncIteratorWithoutYield", resourceCulture);
+            }
+        }
+        
+        /// <summary>
+        ///   Looks up a localized string similar to The body of an async-iterator method must contain a &apos;yield&apos; statement. Consider removing &apos;async&apos; from the method declaration or adding a &apos;yield&apos; statement..
+        /// </summary>
+        internal static string ERR_PossibleAsyncIteratorWithoutYieldOrAwait {
+            get {
+                return ResourceManager.GetString("ERR_PossibleAsyncIteratorWithoutYieldOrAwait", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to To cast a negative value, you must enclose the value in parentheses..
         /// </summary>

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -2641,6 +2641,12 @@ A catch() block after a catch (System.Exception e) block can catch non-CLS excep
   <data name="ERR_AwaitForEachMissingMemberWrongAsync" xml:space="preserve">
     <value>Asynchronous foreach statement cannot operate on variables of type '{0}' because '{0}' does not contain a public instance definition for '{1}'. Did you mean 'foreach' rather than 'await foreach'?</value>
   </data>
+  <data name="ERR_PossibleAsyncIteratorWithoutYield" xml:space="preserve">
+    <value>The body of an async-iterator method must contain a 'yield' statement.</value>
+  </data>
+  <data name="ERR_PossibleAsyncIteratorWithoutYieldOrAwait" xml:space="preserve">
+    <value>The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</value>
+  </data>
   <data name="WRN_BadXMLRefParamType" xml:space="preserve">
     <value>Invalid type for parameter {0} in XML comment cref attribute: '{1}'</value>
   </data>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -1594,6 +1594,8 @@ internal enum ErrorCode
         ERR_BadDynamicAwaitForEach = 8416,
         ERR_NoConvToIAsyncDispWrongAsync = 8417,
         ERR_NoConvToIDispWrongAsync = 8418,
+        ERR_PossibleAsyncIteratorWithoutYield = 8419,
+        ERR_PossibleAsyncIteratorWithoutYieldOrAwait = 8420,
 
         WRN_ConvertingNullableToNonNullable = 8600,
         WRN_NullReferenceAssignment = 8601,

src/Compilers/CSharp/Portable/Lowering/AsyncRewriter/AsyncRewriter.AsyncIteratorRewriter.cs

@@ -36,6 +36,7 @@ private sealed class AsyncIteratorRewriter : AsyncRewriter
                 Debug.Assert(method.IteratorElementType != null);
 
                 _isEnumerable = method.IsIAsyncEnumerableReturningAsync(method.DeclaringCompilation);
+                Debug.Assert(_isEnumerable != method.IsIAsyncEnumeratorReturningAsync(method.DeclaringCompilation));
             }
 
             protected override void VerifyPresenceOfRequiredAPIs(DiagnosticBag bag)

src/Compilers/CSharp/Portable/Lowering/AsyncRewriter/AsyncRewriter.cs

@@ -47,6 +47,19 @@ internal partial class AsyncRewriter : StateMachineRewriter
                 return body;
             }
 
+            CSharpCompilation compilation = method.DeclaringCompilation;
+            bool isAsyncEnumerableOrEnumerator = method.IsIAsyncEnumerableReturningAsync(compilation) ||
+                method.IsIAsyncEnumeratorReturningAsync(compilation);
+            if (isAsyncEnumerableOrEnumerator && !method.IsIterator)
+            {
+                bool containsAwait = AwaitDetector.ContainsAwait(body);
+                diagnostics.Add(containsAwait ? ErrorCode.ERR_PossibleAsyncIteratorWithoutYield : ErrorCode.ERR_PossibleAsyncIteratorWithoutYieldOrAwait,
+                    method.Locations[0], method.ReturnType);
+
+                stateMachineType = null;
+                return body;
+            }
+
             // The CLR doesn't support adding fields to structs, so in order to enable EnC in an async method we need to generate a class.
             // For async-iterators, we also need to generate a class.
             var typeKind = (compilationState.Compilation.Options.EnableEditAndContinue || method.IsIterator) ? TypeKind.Class : TypeKind.Struct;
@@ -56,7 +69,7 @@ internal partial class AsyncRewriter : StateMachineRewriter
             stateMachineType = new AsyncStateMachine(slotAllocatorOpt, compilationState, method, methodOrdinal, typeKind);
             compilationState.ModuleBuilderOpt.CompilationState.SetStateMachineType(method, stateMachineType);
 
-            AsyncRewriter rewriter = method.IsIterator
+            AsyncRewriter rewriter = isAsyncEnumerableOrEnumerator
                 ? new AsyncIteratorRewriter(bodyWithAwaitLifted, method, methodOrdinal, stateMachineType, slotAllocatorOpt, compilationState, diagnostics)
                 : new AsyncRewriter(bodyWithAwaitLifted, method, methodOrdinal, stateMachineType, slotAllocatorOpt, compilationState, diagnostics);
 
@@ -257,5 +270,26 @@ protected virtual void GenerateMoveNext(SynthesizedImplementationMethod moveNext
 
             rewriter.GenerateMoveNext(body, moveNextMethod);
         }
+
+        /// <summary>
+        /// Note: do not use a static/singleton instance of this type, as it holds state.
+        /// </summary>
+        private class AwaitDetector : BoundTreeWalkerWithStackGuardWithoutRecursionOnTheLeftOfBinaryOperator
+        {
+            private bool _sawAwait;
+
+            public static bool ContainsAwait(BoundNode node)
+            {
+                var detector = new AwaitDetector();
+                detector.Visit(node);
+                return detector._sawAwait;
+            }
+
+            public override BoundNode VisitAwaitExpression(BoundAwaitExpression node)
+            {
+                _sawAwait = true;
+                return null;
+            }
+        }
     }
 }

src/Compilers/CSharp/Portable/xlf/CSharpResources.cs.xlf

@@ -152,6 +152,16 @@
         <target state="new">An out variable cannot be declared as a ref local</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYield">
+        <source>The body of an async-iterator method must contain a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYieldOrAwait">
+        <source>The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="new">Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.de.xlf

@@ -152,6 +152,16 @@
         <target state="new">An out variable cannot be declared as a ref local</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYield">
+        <source>The body of an async-iterator method must contain a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYieldOrAwait">
+        <source>The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="new">Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.es.xlf

@@ -152,6 +152,16 @@
         <target state="new">An out variable cannot be declared as a ref local</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYield">
+        <source>The body of an async-iterator method must contain a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYieldOrAwait">
+        <source>The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="new">Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.fr.xlf

@@ -152,6 +152,16 @@
         <target state="new">An out variable cannot be declared as a ref local</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYield">
+        <source>The body of an async-iterator method must contain a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYieldOrAwait">
+        <source>The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="new">Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.it.xlf

@@ -152,6 +152,16 @@
         <target state="new">An out variable cannot be declared as a ref local</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYield">
+        <source>The body of an async-iterator method must contain a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYieldOrAwait">
+        <source>The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="new">Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.ja.xlf

@@ -152,6 +152,16 @@
         <target state="new">An out variable cannot be declared as a ref local</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYield">
+        <source>The body of an async-iterator method must contain a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYieldOrAwait">
+        <source>The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="new">Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.ko.xlf

@@ -152,6 +152,16 @@
         <target state="new">An out variable cannot be declared as a ref local</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYield">
+        <source>The body of an async-iterator method must contain a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYieldOrAwait">
+        <source>The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="new">Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.pl.xlf

@@ -152,6 +152,16 @@
         <target state="new">An out variable cannot be declared as a ref local</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYield">
+        <source>The body of an async-iterator method must contain a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYieldOrAwait">
+        <source>The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="new">Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.pt-BR.xlf

@@ -152,6 +152,16 @@
         <target state="new">An out variable cannot be declared as a ref local</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYield">
+        <source>The body of an async-iterator method must contain a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYieldOrAwait">
+        <source>The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="new">Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.ru.xlf

@@ -152,6 +152,16 @@
         <target state="new">An out variable cannot be declared as a ref local</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYield">
+        <source>The body of an async-iterator method must contain a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYieldOrAwait">
+        <source>The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="new">Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.tr.xlf

@@ -152,6 +152,16 @@
         <target state="new">An out variable cannot be declared as a ref local</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYield">
+        <source>The body of an async-iterator method must contain a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYieldOrAwait">
+        <source>The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="new">Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hans.xlf

@@ -152,6 +152,16 @@
         <target state="new">An out variable cannot be declared as a ref local</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYield">
+        <source>The body of an async-iterator method must contain a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYieldOrAwait">
+        <source>The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="new">Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hant.xlf

@@ -152,6 +152,16 @@
         <target state="new">An out variable cannot be declared as a ref local</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYield">
+        <source>The body of an async-iterator method must contain a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="ERR_PossibleAsyncIteratorWithoutYieldOrAwait">
+        <source>The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</source>
+        <target state="new">The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_RefAssignNarrower">
         <source>Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</source>
         <target state="new">Cannot ref-assign '{1}' to '{0}' because '{1}' has a narrower escape scope than '{0}'.</target>

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

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+using System;
 using System.Linq;
 using System.Text;
 using Microsoft.CodeAnalysis.CSharp.Symbols;
@@ -1200,6 +1201,161 @@ static async System.Collections.Generic.IAsyncEnumerator<int> M(int value)
                 );
         }
 
+        [Fact]
+        [WorkItem(31552, "https://github.com/dotnet/roslyn/issues/31552")]
+        public void AsyncIterator_WithThrowOnly()
+        {
+            string source = @"
+class C
+{
+    public static async System.Collections.Generic.IAsyncEnumerable<int> M()
+    {
+        throw new System.NotImplementedException();
+    }
+}";
+            var comp = CreateCompilationWithAsyncIterator(source);
+            comp.VerifyDiagnostics(
+                // (4,74): warning CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread.
+                //     public static async System.Collections.Generic.IAsyncEnumerable<int> M()
+                Diagnostic(ErrorCode.WRN_AsyncLacksAwaits, "M").WithLocation(4, 74)
+                );
+            comp.VerifyEmitDiagnostics(
+                // (4,74): warning CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread.
+                //     public static async System.Collections.Generic.IAsyncEnumerable<int> M()
+                Diagnostic(ErrorCode.WRN_AsyncLacksAwaits, "M").WithLocation(4, 74),
+                // (4,74): error CS8420: The body of an async-iterator method must contain a 'yield' statement. Consider removing 'async' from the method declaration or adding a 'yield' statement.
+                //     public static async System.Collections.Generic.IAsyncEnumerable<int> M()
+                Diagnostic(ErrorCode.ERR_PossibleAsyncIteratorWithoutYieldOrAwait, "M").WithArguments("System.Collections.Generic.IAsyncEnumerable<int>").WithLocation(4, 74)
+                );
+
+            var m = comp.SourceModule.GlobalNamespace.GetMember<MethodSymbol>("C.M");
+            Assert.False(m.IsIterator);
+            Assert.True(m.IsAsync);
+        }
+
+        [Fact]
+        [WorkItem(31552, "https://github.com/dotnet/roslyn/issues/31552")]
+        public void AsyncIteratorReturningEnumerator_WithThrowOnly()
+        {
+            string source = @"
+class C
+{
+    public static async System.Collections.Generic.IAsyncEnumerator<int> M()
+    {
+        throw new System.NotImplementedException();
+    }
+}";
+            var comp = CreateCompilationWithAsyncIterator(source);
+            comp.VerifyDiagnostics(
+                // (4,74): warning CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread.
+                //     public static async System.Collections.Generic.IAsyncEnumerator<int> M()
+                Diagnostic(ErrorCode.WRN_AsyncLacksAwaits, "M").WithLocation(4, 74)
+                );
+            comp.VerifyEmitDiagnostics(
+                // (4,74): warning CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread.
+                //     public static async System.Collections.Generic.IAsyncEnumerator<int> M()
+                Diagnostic(ErrorCode.WRN_AsyncLacksAwaits, "M").WithLocation(4, 74),
+                // (4,74): error CS8420: The body of an async-iterator method must contain a 'yield' statement. Consider removing `async` from the method declaration.
+                //     public static async System.Collections.Generic.IAsyncEnumerator<int> M()
+                Diagnostic(ErrorCode.ERR_PossibleAsyncIteratorWithoutYieldOrAwait, "M").WithArguments("System.Collections.Generic.IAsyncEnumerator<int>").WithLocation(4, 74)
+                );
+        }
+
+        [Fact]
+        [WorkItem(31552, "https://github.com/dotnet/roslyn/issues/31552")]
+        public void AsyncIteratorReturningEnumerator_WithAwaitAndThrow()
+        {
+            string source = @"
+class C
+{
+    async System.Collections.Generic.IAsyncEnumerator<int> M()
+    {
+        await System.Threading.Tasks.Task.CompletedTask;
+        throw new System.NotImplementedException();
+    }
+}";
+            var comp = CreateCompilationWithAsyncIterator(source);
+            comp.VerifyDiagnostics();
+            comp.VerifyEmitDiagnostics(
+                // (4,60): error CS8419: The body of an async-iterator method must contain a 'yield' statement.
+                //     async System.Collections.Generic.IAsyncEnumerator<int> M()
+                Diagnostic(ErrorCode.ERR_PossibleAsyncIteratorWithoutYield, "M").WithArguments("System.Collections.Generic.IAsyncEnumerator<int>").WithLocation(4, 60)
+                );
+        }
+
+        [Fact]
+        [WorkItem(31552, "https://github.com/dotnet/roslyn/issues/31552")]
+        public void AsyncIteratorReturningEnumerator_WithThrow_WithAwaitInLambda()
+        {
+            string source = @"
+class C
+{
+    async System.Collections.Generic.IAsyncEnumerator<int> M()
+    {
+        System.Func<System.Threading.Tasks.Task> lambda = async () => { await System.Threading.Tasks.Task.CompletedTask; };
+        throw new System.NotImplementedException();
+    }
+}";
+            var comp = CreateCompilationWithAsyncIterator(source);
+            comp.VerifyDiagnostics(
+                // (4,60): warning CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread.
+                //     async System.Collections.Generic.IAsyncEnumerator<int> M()
+                Diagnostic(ErrorCode.WRN_AsyncLacksAwaits, "M").WithLocation(4, 60)
+                );
+            comp.VerifyEmitDiagnostics(
+                // (4,60): warning CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread.
+                //     async System.Collections.Generic.IAsyncEnumerator<int> M()
+                Diagnostic(ErrorCode.WRN_AsyncLacksAwaits, "M").WithLocation(4, 60),
+                // (4,60): error CS8420: The body of an async-iterator method must contain a 'yield' statement. Consider removing `async` from the method declaration.
+                //     async System.Collections.Generic.IAsyncEnumerator<int> M()
+                Diagnostic(ErrorCode.ERR_PossibleAsyncIteratorWithoutYieldOrAwait, "M").WithArguments("System.Collections.Generic.IAsyncEnumerator<int>").WithLocation(4, 60)
+                );
+        }
+
+        [Fact]
+        [WorkItem(31552, "https://github.com/dotnet/roslyn/issues/31552")]
+        public void AsyncIterator_WithEmptyBody()
+        {
+            string source = @"
+class C
+{
+    public static async System.Collections.Generic.IAsyncEnumerable<int> M()
+    {
+    }
+}";
+            var comp = CreateCompilationWithAsyncIterator(source);
+            comp.VerifyDiagnostics(
+                // (4,74): warning CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread.
+                //     public static async System.Collections.Generic.IAsyncEnumerable<int> M()
+                Diagnostic(ErrorCode.WRN_AsyncLacksAwaits, "M").WithLocation(4, 74),
+                // (4,74): error CS0161: 'C.M()': not all code paths return a value
+                //     public static async System.Collections.Generic.IAsyncEnumerable<int> M()
+                Diagnostic(ErrorCode.ERR_ReturnExpected, "M").WithArguments("C.M()").WithLocation(4, 74)
+                );
+        }
+
+        [Fact]
+        [WorkItem(31552, "https://github.com/dotnet/roslyn/issues/31552")]
+        public void AsyncIteratorReturningEnumerator_WithoutBody()
+        {
+            string source = @"
+class C
+{
+    public static async System.Collections.Generic.IAsyncEnumerator<int> M()
+    {
+    }
+}";
+            var comp = CreateCompilationWithAsyncIterator(source);
+            comp.VerifyDiagnostics(
+                // (4,74): warning CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread.
+                //     public static async System.Collections.Generic.IAsyncEnumerable<int> M()
+                Diagnostic(ErrorCode.WRN_AsyncLacksAwaits, "M").WithLocation(4, 74),
+                // (4,74): error CS0161: 'C.M()': not all code paths return a value
+                //     public static async System.Collections.Generic.IAsyncEnumerable<int> M()
+                Diagnostic(ErrorCode.ERR_ReturnExpected, "M").WithArguments("C.M()").WithLocation(4, 74)
+                );
+        }
+
         [ConditionalFact(typeof(WindowsDesktopOnly))]
         [WorkItem(31057, "https://github.com/dotnet/roslyn/issues/31057")]
         public void AsyncIteratorReturningEnumerator_WithoutAwait()