Basic implementation of CS0075. (Issue #13585)

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

@@ -435,6 +435,14 @@ private BoundExpression BindSimpleBinaryOperator(BinaryExpressionSyntax node, Di
 
             if (left.HasAnyErrors || right.HasAnyErrors)
             {
+                if (node.OperatorToken.Kind() == SyntaxKind.MinusToken
+                    && !right.HasAnyErrors
+                    && left.Kind == BoundKind.BadExpression
+                    && left.Type.Kind == SymbolKind.NamedType)
+                {
+                    Error(diagnostics, ErrorCode.ERR_PossibleBadNegCast, node);
+                }
+
                 // NOTE: no user-defined conversion candidates
                 return new BoundBinaryOperator(node, kind, left, right, ConstantValue.NotAvailable, null, LookupResultKind.Empty, GetBinaryOperatorErrorType(kind, diagnostics, node), true);
             }

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

@@ -7459,6 +7459,15 @@ internal class CSharpResources {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to To cast a negative value, you must enclose the value in parentheses..
+        /// </summary>
+        internal static string ERR_PossibleBadNegCast {
+            get {
+                return ResourceManager.GetString("ERR_PossibleBadNegCast", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to Cannot define/undefine preprocessor symbols after first token in file.
         /// </summary>

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -4975,4 +4975,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_ThrowMisplaced" xml:space="preserve">
     <value>A throw expression is not allowed in this context.</value>
   </data>
+  <data name="ERR_PossibleBadNegCast" xml:space="preserve">
+    <value>To cast a negative value, you must enclose the value in parentheses.</value>
+  </data>
 </root>
\ No newline at end of file

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

@@ -70,7 +70,7 @@ internal enum ErrorCode
         ERR_CantOverrideNonEvent = 72,
         ERR_AddRemoveMustHaveBody = 73,
         ERR_AbstractEventInitializer = 74,
-        //ERR_PossibleBadNegCast = 75,      Not used in Roslyn. Occurs so infrequently, not worth reimplementing.
+        ERR_PossibleBadNegCast = 75,
         ERR_ReservedEnumerator = 76,
         ERR_AsMustHaveReferenceType = 77,
         WRN_LowercaseEllSuffix = 78,