Implement improved overload candidates, aka "Bestest Betterness".

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

@@ -259,6 +259,18 @@ private BoundExpression CheckValue(BoundExpression expr, BindValueKind valueKind
             return ToBadExpression(expr, resultKind);
         }
 
+        internal static bool IsTypeOrValueExpression(BoundExpression expression)
+        {
+            switch (expression?.Kind)
+            {
+                case BoundKind.TypeOrValueExpression:
+                case BoundKind.QueryClause when ((BoundQueryClause)expression).Value.Kind == BoundKind.TypeOrValueExpression:
+                    return true;
+                default:
+                    return false;
+            }
+        }
+
         /// <summary>
         /// The purpose of this method is to determine if the expression satisfies desired capabilities. 
         /// If it is not then this code gives an appropriate error message.

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

@@ -347,7 +347,7 @@ internal bool BindingTopLevelScriptCode
             get
             {
                 var containingMember = this.ContainingMemberOrLambda;
-                switch (containingMember.Kind)
+                switch (containingMember?.Kind)
                 {
                     case SymbolKind.Method:
                         // global statements

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

@@ -455,8 +455,7 @@ private static bool IsMethodGroupWithTypeOrValueReceiver(BoundNode node)
                 return false;
             }
 
-            BoundNode receiverOpt = ((BoundMethodGroup)node).ReceiverOpt;
-            return receiverOpt != null && receiverOpt.Kind == BoundKind.TypeOrValueExpression;
+            return Binder.IsTypeOrValueExpression(((BoundMethodGroup)node).ReceiverOpt);
         }
 
         private BoundMethodGroup FixMethodGroupWithTypeOrValue(BoundMethodGroup group, Conversion conversion, DiagnosticBag diagnostics)
@@ -572,7 +571,7 @@ private bool MemberGroupFinalValidationAccessibilityChecks(BoundExpression recei
             //note that the same assert does not hold for all properties. Some properties and (all indexers) are not referenceable by name, yet
             //their binding brings them through here, perhaps needlessly.
 
-            if (receiverOpt != null && receiverOpt.Kind == BoundKind.TypeOrValueExpression)
+            if (IsTypeOrValueExpression(receiverOpt))
             {
                 // TypeOrValue expression isn't replaced only if the invocation is late bound, in which case it can't be extension method.
                 // None of the checks below apply if the receiver can't be classified as a type or value. 
@@ -678,7 +677,7 @@ private static bool IsMemberAccessedThroughVariableOrValue(BoundExpression recei
             return !IsMemberAccessedThroughType(receiverOpt);
         }
 
-        private static bool IsMemberAccessedThroughType(BoundExpression receiverOpt)
+        internal static bool IsMemberAccessedThroughType(BoundExpression receiverOpt)
         {
             if (receiverOpt == null)
             {
@@ -696,7 +695,7 @@ private static bool IsMemberAccessedThroughType(BoundExpression receiverOpt)
         /// <summary>
         /// Was the receiver expression compiler-generated?
         /// </summary>
-        private static bool WasImplicitReceiver(BoundExpression receiverOpt)
+        internal static bool WasImplicitReceiver(BoundExpression receiverOpt)
         {
             if (receiverOpt == null) return true;
             if (!receiverOpt.WasCompilerGenerated) return false;
@@ -808,8 +807,8 @@ private bool MethodGroupConversionHasErrors(
 
             MethodSymbol selectedMethod = conversion.Method;
 
-            if (MemberGroupFinalValidation(receiverOpt, selectedMethod, syntax, diagnostics, isExtensionMethod) ||
-                !MethodGroupIsCompatibleWithDelegate(receiverOpt, isExtensionMethod, selectedMethod, delegateType, syntax.Location, diagnostics))
+            if (!MethodGroupIsCompatibleWithDelegate(receiverOpt, isExtensionMethod, selectedMethod, delegateType, syntax.Location, diagnostics) ||
+                MemberGroupFinalValidation(receiverOpt, selectedMethod, syntax, diagnostics, isExtensionMethod))
             {
                 return true;
             }
@@ -864,8 +863,12 @@ private bool MethodGroupConversionDoesNotExistOrHasErrors(
             diagnostics.Add(delegateMismatchLocation, useSiteDiagnostics);
             if (!conversion.Exists)
             {
-                // No overload for '{0}' matches delegate '{1}'
-                diagnostics.Add(ErrorCode.ERR_MethDelegateMismatch, delegateMismatchLocation, boundMethodGroup.Name, delegateType);
+                if (!Conversions.ReportDelegateMethodGroupDiagnostics(this, boundMethodGroup, delegateType, diagnostics))
+                {
+                    // No overload for '{0}' matches delegate '{1}'
+                    diagnostics.Add(ErrorCode.ERR_MethDelegateMismatch, delegateMismatchLocation, boundMethodGroup.Name, delegateType);
+                }
+
                 return true;
             }
             else