Disallow ref like types as iterator element types.

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

@@ -147,6 +147,10 @@ public static void ValidateIteratorMethod(CSharpCompilation compilation, MethodS
                     Error(diagnostics, ErrorCode.ERR_BadIteratorReturn, errorLocation, iterator, returnType);
                 }
             }
+            else if (elementType.IsRefLikeOrAllowsRefLikeType())
+            {
+                Error(diagnostics, ErrorCode.ERR_IteratorRefLikeElementType, errorLocation);
+            }
 
             bool asyncInterface = InMethodBinder.IsAsyncStreamInterface(compilation, refKind, returnType);
             if (asyncInterface && !iterator.IsAsync)

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -8014,4 +8014,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="WRN_AccessorDoesNotUseBackingField_Title" xml:space="preserve">
     <value>Property accessor should use 'field' because the other accessor is using it.</value>
   </data>
+  <data name="ERR_IteratorRefLikeElementType" xml:space="preserve">
+    <value>Element type of an iterator may not be a ref struct or a type parameter allowing ref structs</value>
+  </data>
 </root>

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

@@ -2349,6 +2349,8 @@ internal enum ErrorCode
         WRN_UnassignedInternalRefField = 9265,
         WRN_AccessorDoesNotUseBackingField = 9266,
 
+        ERR_IteratorRefLikeElementType = 9267,
+
         // Note: you will need to do the following after adding errors:
         //  1) Update ErrorFacts.IsBuildOnlyDiagnostic (src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs)
 

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

@@ -2466,6 +2466,7 @@ or ErrorCode.ERR_PartialPropertyDuplicateInitializer
                 or ErrorCode.WRN_UninitializedNonNullableBackingField
                 or ErrorCode.WRN_UnassignedInternalRefField
                 or ErrorCode.WRN_AccessorDoesNotUseBackingField
+                or ErrorCode.ERR_IteratorRefLikeElementType
                     => false,
             };
 #pragma warning restore CS8524 // The switch expression does not handle some values of its input type (it is not exhaustive) involving an unnamed enum value.

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

@@ -641,7 +641,10 @@ ref struct S
             {
                 // source(4,65): error CS9244: The type 'S' may not be a ref struct or a type parameter allowing ref structs in order to use it as parameter 'T' in the generic type or method 'IAsyncEnumerable<T>'
                 //     static async System.Collections.Generic.IAsyncEnumerable<S> M()
-                Diagnostic(ErrorCode.ERR_NotRefStructConstraintNotSatisfied, "M").WithArguments("System.Collections.Generic.IAsyncEnumerable<T>", "T", "S").WithLocation(4, 65)
+                Diagnostic(ErrorCode.ERR_NotRefStructConstraintNotSatisfied, "M").WithArguments("System.Collections.Generic.IAsyncEnumerable<T>", "T", "S").WithLocation(4, 65),
+                // source(4,65): error CS9267: Element type of an iterator may not be a ref struct or a type parameter allowing ref structs
+                //     static async System.Collections.Generic.IAsyncEnumerable<S> M()
+                Diagnostic(ErrorCode.ERR_IteratorRefLikeElementType, "M").WithLocation(4, 65)
             };
 
             var comp = CreateCompilationWithAsyncIterator(source, options: TestOptions.DebugExe);
@@ -655,6 +658,9 @@ ref struct S
                 // source(4,65): error CS9244: The type 'S' may not be a ref struct or a type parameter allowing ref structs in order to use it as parameter 'T' in the generic type or method 'IAsyncEnumerable<T>'
                 //     static async System.Collections.Generic.IAsyncEnumerable<S> M()
                 Diagnostic(ErrorCode.ERR_NotRefStructConstraintNotSatisfied, "M").WithArguments("System.Collections.Generic.IAsyncEnumerable<T>", "T", "S").WithLocation(4, 65),
+                // source(4,65): error CS9267: Element type of an iterator may not be a ref struct or a type parameter allowing ref structs
+                //     static async System.Collections.Generic.IAsyncEnumerable<S> M()
+                Diagnostic(ErrorCode.ERR_IteratorRefLikeElementType, "M").WithLocation(4, 65),
                 // source(11,24): error CS9202: Feature 'ref and unsafe in async and iterator methods' is not available in C# 12.0. Please use language version 13.0 or greater.
                 //         await foreach (var s in M())
                 Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion12, "var").WithArguments("ref and unsafe in async and iterator methods", "13.0").WithLocation(11, 24));