Implements #6061

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

@@ -11711,7 +11711,7 @@ internal class CSharpResources {
         }
         
         /// <summary>
-        ///   Looks up a localized string similar to &apos;{0}&apos; is not a recognized attribute location. All attributes in this block will be ignored..
+        ///   Looks up a localized string similar to &apos;{0}&apos; is not a recognized attribute location. Valid attribute locations for this declaration are &apos;{1}&apos;. All attributes in this block will be ignored..
         /// </summary>
         internal static string WRN_InvalidAttributeLocation {
             get {

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -1758,7 +1758,7 @@ If such a class is used as a base class and if the deriving class defines a dest
     <value>Not a valid attribute location for this declaration</value>
   </data>
   <data name="WRN_InvalidAttributeLocation" xml:space="preserve">
-    <value>'{0}' is not a recognized attribute location. All attributes in this block will be ignored.</value>
+    <value>'{0}' is not a recognized attribute location. Valid attribute locations for this declaration are '{1}'. All attributes in this block will be ignored.</value>
   </data>
   <data name="WRN_InvalidAttributeLocation_Title" xml:space="preserve">
     <value>Not a recognized attribute location</value>

src/Compilers/CSharp/Portable/Symbols/Symbol_Attributes.cs

@@ -462,6 +462,8 @@ private static bool MatchAttributeTarget(IAttributeTargetSymbol attributeTarget,
                 return isOwner;
             }
 
+            AttributeLocation allowedTargets = attributesOwner.AllowedAttributeLocations;
+
             AttributeLocation explicitTarget = targetOpt.GetAttributeLocation();
             if (explicitTarget == AttributeLocation.None)
             {
@@ -470,13 +472,12 @@ private static bool MatchAttributeTarget(IAttributeTargetSymbol attributeTarget,
                 {
                     //NOTE: ValueText so that we accept targets like "@return", to match dev10 (DevDiv #2591).
                     diagnostics.Add(ErrorCode.WRN_InvalidAttributeLocation,
-                        targetOpt.Identifier.GetLocation(), targetOpt.Identifier.ValueText);
+                        targetOpt.Identifier.GetLocation(), targetOpt.Identifier.ValueText, allowedTargets.ToDisplayString());
                 }
 
                 return false;
             }
             
-            AttributeLocation allowedTargets = attributesOwner.AllowedAttributeLocations;
             if ((explicitTarget & allowedTargets) == 0)
             {
                 // error: invalid target for symbol