Warn when taking address of local or parameter in async method

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

@@ -817,6 +817,11 @@ private static bool CheckNotNamespaceOrType(BoundExpression expr, BindingDiagnos
 
         private bool CheckLocalValueKind(SyntaxNode node, BoundLocal local, BindValueKind valueKind, bool checkingReceiver, BindingDiagnosticBag diagnostics)
         {
+            if (valueKind == BindValueKind.AddressOf && this.IsInAsyncMethod())
+            {
+                Error(diagnostics, ErrorCode.WRN_AddressOfInAsync, node);
+            }
+
             // Local constants are never variables. Local variables are sometimes
             // not to be treated as variables, if they are fixed, declared in a using, 
             // or declared in a foreach.
@@ -906,6 +911,10 @@ internal partial class Binder
         private bool CheckParameterValueKind(SyntaxNode node, BoundParameter parameter, BindValueKind valueKind, bool checkingReceiver, BindingDiagnosticBag diagnostics)
         {
             Debug.Assert(!RequiresAssignableVariable(BindValueKind.AddressOf));
+            if (valueKind == BindValueKind.AddressOf && this.IsInAsyncMethod())
+            {
+                Error(diagnostics, ErrorCode.WRN_AddressOfInAsync, node);
+            }
 
             ParameterSymbol parameterSymbol = parameter.ParameterSymbol;
 

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -7493,4 +7493,10 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_UnexpectedParameterList" xml:space="preserve">
     <value>Unexpected parameter list.</value>
   </data>
+  <data name="WRN_AddressOfInAsync" xml:space="preserve">
+    <value>The '&amp;' operator should not be used on parameters or local variables in async methods.</value>
+  </data>
+  <data name="WRN_AddressOfInAsync_Title" xml:space="preserve">
+    <value>The '&amp;' operator should not be used on parameters or local variables in async methods.</value>
+  </data>
 </root>

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

@@ -2181,11 +2181,11 @@ internal enum ErrorCode
         ERR_RefReturnPrimaryConstructorParameter = 9120,
         ERR_StructLayoutCyclePrimaryConstructorParameter = 9121,
         ERR_UnexpectedParameterList = 9122,
+        WRN_AddressOfInAsync = 9123,
 
         ERR_BadRefInUsingAlias = 9130,
         ERR_BadUnsafeInUsingDirective = 9131,
         ERR_BadNullableReferenceTypeInUsingAlias = 9132,
-
         #endregion
 
         // Note: you will need to do the following after adding warnings:

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

@@ -206,6 +206,10 @@ internal static int GetWarningLevel(ErrorCode code)
             // docs/compilers/CSharp/Warnversion Warning Waves.md
             switch (code)
             {
+                case ErrorCode.WRN_AddressOfInAsync:
+                    // Warning level 8 is exclusively for warnings introduced in the compiler
+                    // shipped with dotnet 8 (C# 12) and that can be reported for pre-existing code.
+                    return 8;
                 case ErrorCode.WRN_LowerCaseTypeName:
                     // Warning level 7 is exclusively for warnings introduced in the compiler
                     // shipped with dotnet 7 (C# 11) and that can be reported for pre-existing code.
@@ -2299,6 +2303,7 @@ internal static bool IsBuildOnlyDiagnostic(ErrorCode code)
                 case ErrorCode.ERR_RefReturnPrimaryConstructorParameter:
                 case ErrorCode.ERR_StructLayoutCyclePrimaryConstructorParameter:
                 case ErrorCode.ERR_UnexpectedParameterList:
+                case ErrorCode.WRN_AddressOfInAsync:
                 case ErrorCode.ERR_BadRefInUsingAlias:
                 case ErrorCode.ERR_BadUnsafeInUsingDirective:
                 case ErrorCode.ERR_BadNullableReferenceTypeInUsingAlias: