Add more specific diagnostic message for conditional-access on unconstrained generic

Author: RikkiGibson

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

@@ -11496,7 +11496,10 @@ internal static bool MethodOrLambdaRequiresValue(Symbol symbol, CSharpCompilatio
 
         private BoundConditionalAccess GenerateBadConditionalAccessNodeError(ConditionalAccessExpressionSyntax node, BoundExpression receiver, BoundExpression access, BindingDiagnosticBag diagnostics)
         {
-            DiagnosticInfo diagnosticInfo = new CSDiagnosticInfo(ErrorCode.ERR_CannotBeMadeNullable, access.Display);
+            var errorCode = !accessType.IsReferenceType && !accessType.IsValueType
+                ? ErrorCode.ERR_ConditionalAccessNotReferenceOrValueType
+                : ErrorCode.ERR_CannotBeMadeNullable;
+            DiagnosticInfo diagnosticInfo = new CSDiagnosticInfo(errorCode, access.Display);
             diagnostics.Add(new CSDiagnostic(diagnosticInfo, access.Syntax.Location));
             receiver = BadExpression(receiver.Syntax, receiver);
 

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -8183,4 +8183,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_AmbigOperator" xml:space="preserve">
     <value>Operator resolution is ambiguous between the following members: '{0}' and '{1}'</value>
   </data>
+  <data name="ERR_ConditionalAccessNotReferenceTypeOrValueType" xml:space="preserve">
+    <value>Cannot use the result of a conditional access of type '{0}' because 'null' cannot be converted to '{0}?'.</value>
+  </data>
 </root>

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

@@ -2441,6 +2441,7 @@ internal enum ErrorCode
         ERR_AmbigOperator = 9342,
 
         ERR_UnexpectedArgumentListInBaseTypeWithoutParameterList = 9343,
+        ERR_ConditionalAccessNotReferenceOrValueType = 9344,
 
         // 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

@@ -2548,6 +2548,7 @@ or ErrorCode.ERR_SingleInapplicableBinaryOperator
                 or ErrorCode.ERR_SingleInapplicableUnaryOperator
                 or ErrorCode.ERR_AmbigOperator
                 or ErrorCode.ERR_UnexpectedArgumentListInBaseTypeWithoutParameterList
+                or ErrorCode.ERR_ConditionalAccessNotReferenceOrValueType
                     => 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.