Warning if /refout and /out filenames don't match

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -5060,6 +5060,12 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_VoidAssignment" xml:space="preserve">
     <value>A value of type 'void' may not be assigned.</value>
   </data>
+  <data name="WRN_RefOutFilenameDoesNotMatchOut" xml:space="preserve">
+    <value>The filenames for the main output '{0}' and reference output '{1}' don't match. That is not recommended.</value>
+  </data>
+  <data name="WRN_RefOutFilenameDoesNotMatchOut_Title" xml:space="preserve">
+    <value>The filenames for the main output and reference output don't match. That is not recommended.</value>
+  </data>
   <data name="WRN_Experimental" xml:space="preserve">
     <value>'{0}' is for evaluation purposes only and is subject to change or removal in future updates.</value>
   </data>

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

@@ -1481,7 +1481,7 @@ internal enum ErrorCode
 
         ERR_NoRefOutWhenRefOnly = 8308,
         ERR_NoNetModuleOutputWhenRefOutOrRefOnly = 8309,
-        // Available = 8310,
+        WRN_RefOutFilenameDoesNotMatchOut = 8310,
         ERR_BadDynamicMethodArgDefaultLiteral = 8311,
         ERR_DefaultLiteralNotValid = 8312,
         WRN_DefaultInSwitch = 8313,

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

@@ -319,6 +319,7 @@ internal static int GetWarningLevel(ErrorCode code)
                 case ErrorCode.WRN_TupleLiteralNameMismatch:
                 case ErrorCode.WRN_Experimental:
                 case ErrorCode.WRN_DefaultInSwitch:
+                case ErrorCode.WRN_RefOutFilenameDoesNotMatchOut:
                     return 1;
                 default:
                     return 0;

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

@@ -172,6 +172,9 @@ public override ReportDiagnostic GetDiagnosticReport(DiagnosticInfo diagnosticIn
         public override int ERR_MetadataReferencesNotSupported => (int)ErrorCode.ERR_MetadataReferencesNotSupported;
         public override int ERR_LinkedNetmoduleMetadataMustProvideFullPEImage => (int)ErrorCode.ERR_LinkedNetmoduleMetadataMustProvideFullPEImage;
 
+        // refout:
+        public override int WRN_RefOutFilenameDoesNotMatchOut => (int)ErrorCode.WRN_RefOutFilenameDoesNotMatchOut;
+
         public override void ReportDuplicateMetadataReferenceStrong(DiagnosticBag diagnostics, Location location, MetadataReference reference, AssemblyIdentity identity, MetadataReference equivalentReference, AssemblyIdentity equivalentIdentity)
         {
             diagnostics.Add(ErrorCode.ERR_DuplicateImport, location,

src/Compilers/CSharp/Portable/Generated/ErrorFacts.Generated.cs

@@ -174,6 +174,7 @@ public static bool IsWarning(ErrorCode code)
                 case ErrorCode.WRN_AttributeIgnoredWhenPublicSigning:
                 case ErrorCode.WRN_TupleLiteralNameMismatch:
                 case ErrorCode.WRN_Experimental:
+                case ErrorCode.WRN_RefOutFilenameDoesNotMatchOut:
                 case ErrorCode.WRN_DefaultInSwitch:
                     return true;
                 default:

src/Compilers/CSharp/Test/CommandLine/CommandLineTests.cs

@@ -9130,6 +9130,33 @@ private static void PrivateMethod()
             CleanupAllGeneratedFiles(refDir.Path);
         }
 
+        [Fact]
+        public void RefOutWithFilenameMismatch()
+        {
+            var dir = Temp.CreateDirectory();
+            var refDir = dir.CreateDirectory("ref");
+
+            var src = dir.CreateFile("a.cs");
+            src.WriteAllText(@"
+public class C
+{
+}");
+
+            var outWriter = new StringWriter(CultureInfo.InvariantCulture);
+            var csc = new MockCSharpCompiler(null, dir.Path,
+                new[] { "/nologo", "/refout:ref/b.dll", "/target:library", "/deterministic", "a.cs" });
+
+            int exitCode = csc.Run(outWriter);
+
+            Assert.Equal("error CS8310: The filenames for the main output 'a.dll' and reference output 'b.dll' don't match. That is not recommended.",
+                outWriter.ToString().Trim());
+            Assert.Equal(0, exitCode);
+
+            // Clean up temp files
+            CleanupAllGeneratedFiles(dir.Path);
+            CleanupAllGeneratedFiles(refDir.Path);
+        }
+
         [Fact]
         public void RefOutWithError()
         {

src/Compilers/Core/Portable/CommandLine/CommonCompiler.cs

@@ -752,6 +752,7 @@ private int RunCore(TextWriter consoleOutput, ErrorLogger errorLogger, Cancellat
                             var pdbStreamProviderOpt = emitPdbFile ? new CompilerEmitStreamProvider(this, finalPdbFilePath) : null;
 
                             string finalRefPeFilePath = Arguments.OutputRefFilePath;
+                            ReportRefFilenameMismatch(finalPeFilePath, finalRefPeFilePath, diagnosticBag);
                             var refPeStreamProviderOpt = finalRefPeFilePath != null ? new CompilerEmitStreamProvider(this, finalRefPeFilePath) : null;
 
                             try
@@ -861,6 +862,22 @@ private int RunCore(TextWriter consoleOutput, ErrorLogger errorLogger, Cancellat
             return Succeeded;
         }
 
+        private void ReportRefFilenameMismatch(string finalPeFilePath, string finalRefPeFilePath, DiagnosticBag diagnosticBag)
+        {
+            if (finalPeFilePath == null || finalRefPeFilePath == null)
+            {
+                return;
+            }
+
+            var primaryOutput = Path.GetFileName(finalPeFilePath);
+            var secondaryOutput = Path.GetFileName(finalRefPeFilePath);
+            if (!primaryOutput.Equals(secondaryOutput, StringComparison.OrdinalIgnoreCase))
+            {
+                diagnosticBag.Add(MessageProvider.CreateDiagnostic(MessageProvider.WRN_RefOutFilenameDoesNotMatchOut,
+                    Location.None, primaryOutput, secondaryOutput));
+            }
+        }
+
         private bool WriteTouchedFiles(TextWriter consoleOutput, TouchedFileLogger touchedFilesLogger, string finalXmlFilePath)
         {
             if (Arguments.TouchedFilesPath != null)

src/Compilers/Core/Portable/Diagnostic/CommonMessageProvider.cs

@@ -221,6 +221,9 @@ public DiagnosticInfo FilterDiagnosticInfo(DiagnosticInfo diagnosticInfo, Compil
         public abstract int ERR_ModuleEmitFailure { get; }
         public abstract int ERR_EncUpdateFailedMissingAttribute { get; }
 
+        // Refout:
+        public abstract int WRN_RefOutFilenameDoesNotMatchOut { get; }
+
         /// <summary>
         /// Takes an exception produced while writing to a file stream and produces a diagnostic.
         /// </summary>

src/Compilers/VisualBasic/Portable/Errors/Errors.vb

@@ -1955,9 +1955,10 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
 
         WRN_AttributeIgnoredWhenPublicSigning = 42379
         WRN_Experimental = 42380
+        WRN_RefOutFilenameDoesNotMatchOut = 42381
 
-        ' // AVAILABLE                             42381 - 49998
-        ERRWRN_Last = WRN_Experimental
+        ' // AVAILABLE                             42382 - 49998
+        ERRWRN_Last = WRN_RefOutFilenameDoesNotMatchOut
 
         '// HIDDENS AND INFOS BEGIN HERE
         HDN_UnusedImportClause = 50000

src/Compilers/VisualBasic/Portable/Errors/MessageProvider.vb

@@ -213,6 +213,12 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
             End Get
         End Property
 
+        Public Overrides ReadOnly Property WRN_RefOutFilenameDoesNotMatchOut As Integer
+            Get
+                Return ERRID.WRN_RefOutFilenameDoesNotMatchOut
+            End Get
+        End Property
+
         Public Overrides ReadOnly Property INF_UnableToLoadSomeTypesInAnalyzer As Integer
             Get
                 Return ERRID.INF_UnableToLoadSomeTypesInAnalyzer

src/Compilers/VisualBasic/Portable/Generated/ErrorFacts.Generated.vb

@@ -169,7 +169,8 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
                      ERRID.WRN_NoAnalyzerInAssembly,
                      ERRID.WRN_UnableToLoadAnalyzer,
                      ERRID.WRN_AttributeIgnoredWhenPublicSigning,
-                     ERRID.WRN_Experimental
+                     ERRID.WRN_Experimental,
+                     ERRID.WRN_RefOutFilenameDoesNotMatchOut
                     Return True
                 Case Else
                     Return False

src/Compilers/VisualBasic/Portable/VBResources.resx

@@ -5492,4 +5492,10 @@
   <data name="WRN_Experimental_Title" xml:space="preserve">
     <value>Type is for evaluation purposes only and is subject to change or removal in future updates.</value>
   </data>
+  <data name="WRN_RefOutFilenameDoesNotMatchOut" xml:space="preserve">
+    <value>The filenames for the main output '{0}' and reference output '{1}' don't match. That is not recommended.</value>
+  </data>
+  <data name="WRN_RefOutFilenameDoesNotMatchOut_Title" xml:space="preserve">
+    <value>The filenames for the main output and reference output don't match. That is not recommended.</value>
+  </data>
 </root>

src/Compilers/VisualBasic/Test/CommandLine/CommandLineTests.vb

@@ -8252,6 +8252,30 @@ a
             CleanupAllGeneratedFiles(refDir.Path)
         End Sub
 
+        <Fact>
+        Public Sub RefOutWithFilenameMismatch()
+            Dim dir = Temp.CreateDirectory()
+            Dim refDir = dir.CreateDirectory("ref")
+
+            Dim src = dir.CreateFile("a.vb")
+            src.WriteAllText("
+Public Class C
+End Class")
+
+            Dim outWriter = New StringWriter(CultureInfo.InvariantCulture)
+            Dim vbc = New MockVisualBasicCompiler(Nothing, dir.Path,
+                {"/define:_MYTYPE=""Empty"" ", "/nologo", "/refout:ref/b.dll", "/target:library", "/deterministic", "a.vb"})
+
+            Dim exitCode = vbc.Run(outWriter)
+            Assert.Equal("vbc : warning BC42381: The filenames for the main output 'a.dll' and reference output 'b.dll' don't match. That is not recommended.",
+                outWriter.ToString().Trim())
+            Assert.Equal(0, exitCode)
+
+            ' Clean up temp files
+            CleanupAllGeneratedFiles(dir.Path)
+            CleanupAllGeneratedFiles(refDir.Path)
+        End Sub
+
         <Fact>
         Public Sub RefOutWithError()
             Dim dir = Temp.CreateDirectory()

src/Test/Utilities/Portable/Mocks/TestMessageProvider.cs

@@ -393,15 +393,15 @@ public override int ERR_TooManyUserStrings
                 throw new NotImplementedException();
             }
         }
-        
+
         public override int ERR_PeWritingFailure
         {
             get
             {
                 throw new NotImplementedException();
             }
         }
-        
+
         public override int ERR_ModuleEmitFailure
         {
             get
@@ -441,5 +441,13 @@ public override int ERR_BadDocumentationMode
                 throw new NotImplementedException();
             }
         }
+
+        public override int WRN_RefOutFilenameDoesNotMatchOut
+        {
+            get
+            {
+                throw new NotImplementedException();
+            }
+        }
     }
 }