Deduplicate module names in FCS

src/fsharp/vs/IncrementalBuild.fs

@@ -1328,6 +1328,14 @@ type IncrementalBuilder(tcGlobals,frameworkTcImports, nonFrameworkAssemblyInputs
     let StampFileNameTask (cache: TimeStampCache) _ctok (_m:range, filename:string, _isLastCompiland) =
         assertNotDisposed()
         cache.GetFileTimeStamp filename
+
+    // Deduplicate module names
+    let seen = Dictionary<string,Set<string>>()
+    let deduplicate (paths: Set<string>) path (qualifiedNameOfFile: QualifiedNameOfFile) =
+        let count = if paths.Contains path then paths.Count else paths.Count + 1
+        seen.[qualifiedNameOfFile.Text] <- Set.add path paths
+        let id = qualifiedNameOfFile.Id
+        if count = 1 then qualifiedNameOfFile else QualifiedNameOfFile(Ident(id.idText + "___" + count.ToString(),id.idRange))
 
     /// This is a build task function that gets placed into the build rules as the computation for a VectorMap
     ///
@@ -1344,8 +1352,31 @@ type IncrementalBuilder(tcGlobals,frameworkTcImports, nonFrameworkAssemblyInputs
 
         try  
             IncrementalBuilderEventTesting.MRU.Add(IncrementalBuilderEventTesting.IBEParsed filename)
-            let result = ParseOneInputFile(tcConfig,lexResourceManager, [], filename ,isLastCompiland,errorLogger,(*retryLocked*)true)
+            let input = ParseOneInputFile(tcConfig,lexResourceManager, [], filename ,isLastCompiland,errorLogger,(*retryLocked*)true)
             fileParsed.Trigger (filename)
+            let result =
+                match input with
+                | Some(ParsedInput.ImplFile (ParsedImplFileInput.ParsedImplFileInput(fileName,isScript,qualifiedNameOfFile,scopedPragmas,hashDirectives,modules,(isLastCompiland,isExe)))) ->
+                    let path = Path.GetDirectoryName fileName
+                    match seen.TryGetValue qualifiedNameOfFile.Text with
+                    | true, paths ->
+                        let qualifiedNameOfFile = deduplicate paths path qualifiedNameOfFile
+                        let input = ParsedInput.ImplFile(ParsedImplFileInput.ParsedImplFileInput(fileName,isScript,qualifiedNameOfFile,scopedPragmas,hashDirectives,modules,(isLastCompiland,isExe)))
+                        Some input
+                    | _ ->
+                        seen.Add(qualifiedNameOfFile.Text,Set.singleton path)
+                        input
+                | Some(ParsedInput.SigFile (ParsedSigFileInput.ParsedSigFileInput(fileName,qualifiedNameOfFile,scopedPragmas,hashDirectives,modules))) ->
+                    let path = Path.GetDirectoryName fileName
+                    match seen.TryGetValue qualifiedNameOfFile.Text with
+                    | true, paths ->
+                        let qualifiedNameOfFile = deduplicate paths path qualifiedNameOfFile
+                        let input = ParsedInput.SigFile (ParsedSigFileInput.ParsedSigFileInput(fileName,qualifiedNameOfFile,scopedPragmas,hashDirectives,modules))
+                        Some input
+                    | _ ->
+                        seen.Add(qualifiedNameOfFile.Text,Set.singleton path)
+                        input
+                | input -> input
             result,sourceRange,filename,errorLogger.GetErrors ()
         with exn -> 
             System.Diagnostics.Debug.Assert(false, sprintf "unexpected failure in IncrementalFSharpBuild.Parse\nerror = %s" (exn.ToString()))

tests/service/ProjectAnalysisTests.fs

@@ -4851,3 +4851,21 @@ let ``Test request for parse and check doesn't check whole project`` () =
 
     ()
 
+[<Test>]
+// Simplified repro for https://github.com/Microsoft/visualfsharp/issues/2679
+let ``add files with same name from different folders`` () = 
+    let fileNames =
+        [ __SOURCE_DIRECTORY__ + "/data/samename/folder1/a.fs"
+          __SOURCE_DIRECTORY__ + "/data/samename/folder2/a.fs" ]
+    let projFileName = __SOURCE_DIRECTORY__ + "/data/samename/tempet.fsproj"
+    let args = mkProjectCommandLineArgs ("test.dll", fileNames)
+    let options = checker.GetProjectOptionsFromCommandLineArgs (projFileName, args)
+    let wholeProjectResults = checker.ParseAndCheckProject(options) |> Async.RunSynchronously
+    let errors =
+        wholeProjectResults.Errors
+        |> Array.filter (fun x -> x.Severity = FSharpErrorSeverity.Error)
+    if errors.Length > 0 then
+        printfn "add files with same name from different folders"
+        for err in errors do
+            printfn "ERROR: %s" err.Message
+    shouldEqual 0 errors.Length

tests/service/data/samename/folder1/a.fs

@@ -0,0 +1,5 @@
+namespace tempet
+
+module SayA =
+    let hello name =
+        printfn "Hello %s" name

tests/service/data/samename/folder2/a.fs

@@ -0,0 +1,5 @@
+namespace tempet
+
+module SayB =
+    let hello name =
+        printfn "Hello %s" name

tests/service/data/samename/tempet.fsproj

@@ -0,0 +1,13 @@
+<Project Sdk="FSharp.NET.Sdk;Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>netstandard1.6</TargetFramework>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="folder1/a.fs" />
+    <Compile Include="folder2/a.fs" />
+  </ItemGroup>
+  <ItemGroup>
+    <PackageReference Include="FSharp.Core" Version="4.1.*" />
+    <PackageReference Include="FSharp.NET.Sdk" Version="1.0.*" PrivateAssets="All" />
+  </ItemGroup>
+</Project>