Merge pull request #28206 from OmarTawfik/bugs/28117/ref-assign-to-parameters

Produce errors on ref-assignment to byval parameters

docs/compilers/CSharp/Compiler Breaking Changes - post VS2017.md

@@ -136,3 +136,4 @@ Example:
 - Visual Studio 2017 version 15.8: https://github.com/dotnet/roslyn/pull/27803 pattern matching now will produce errors when trying to return a stack bound value to an invalid escape scope.
 - Visual Studio 2017 version 15.8: https://github.com/dotnet/roslyn/issues/26743 C# will now reject expressions such as `public int* M => &this.Bar[0];` if `Bar` is a fixed field of the containing type.
 - Visual Studio 2017 version 15.8: https://github.com/dotnet/roslyn/issues/27772 invocation receivers will be checked for escape scope errors now in nested scope expressions.
+- Visual Studio 2017 version 15.8: https://github.com/dotnet/roslyn/issues/28117 C# will now reject ref assignments to byval parameters.

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

@@ -588,6 +588,11 @@ private bool CheckParameterValueKind(SyntaxNode node, BoundParameter parameter,
                 ReportReadOnlyError(parameterSymbol, node, valueKind, checkingReceiver, diagnostics);
                 return false;
             }
+            else if (parameterSymbol.RefKind == RefKind.None && RequiresRefAssignableVariable(valueKind))
+            {
+                Error(diagnostics, ErrorCode.ERR_RefLocalOrParamExpected, node);
+                return false;
+            }
 
             if (this.LockedOrDisposedVariables.Contains(parameterSymbol))
             {

src/Compilers/CSharp/Test/Semantic/Semantics/RefLocalsAndReturnsTests.cs

@@ -3818,6 +3818,23 @@ void N()
                 Diagnostic(ErrorCode.ERR_SyntaxError, "x").WithArguments(",", "").WithLocation(10, 22));
         }
 
+        [Fact]
+        [WorkItem(28117, "https://github.com/dotnet/roslyn/issues/28117")]
+        public void AssigningRefToParameter()
+        {
+            CreateCompilation(@"
+public class C
+{
+    void M(int a, ref int b)
+    {
+        a = ref b;
+    }
+}").VerifyDiagnostics(
+                // (6,9): error CS8373: The left-hand side of a ref assignment must be a ref local or parameter.
+                //         a = ref b;
+                Diagnostic(ErrorCode.ERR_RefLocalOrParamExpected, "a").WithLocation(6, 9));
+        }
+
         [Fact]
         [WorkItem(26516, "https://github.com/dotnet/roslyn/issues/26516")]
         public void BindingRefVoidAssignment()