FSharp.Compiler.Service v34.0 seems to have had a breaking API change

Description

After updating a Fake script to v5.19.1 from v5.19 (which updated from FSharp.Compiler.Service (33.0.1) to FSharp.Compiler.Service (34.0.1)) and incidentally FSharpLint.Core from 0.12.5 to 0.12.10, the linting operation failed with exception

System.MissingMethodException: Method not found: 'FSharp.Compiler.SourceCodeServices.FSharpChecker FSharp.Compiler.SourceCodeServices.FSharpChecker.Create(Microsoft.FSharp.Core.FSharpOption`1<Int32>, Microsoft.FSharp.Core.FSharpOption`1<Boolean>, Microsoft.FSharp.Core.FSharpOption`1<Boolean>, Microsoft.FSharp.Core.FSharpOption`1<Resolver>, Microsoft.FSharp.Core.FSharpOption`1<Microsoft.FSharp.Core.FSharpFunc`2<System.Tuple`2<System.String,System.DateTime>,Microsoft.FSharp.Core.FSharpOption`1<System.Tuple`3<System.Object,IntPtr,Int32>>>>, Microsoft.FSharp.Core.FSharpOption`1<Boolean>)'.
           at FSharpLint.Application.Lint.lintFile(OptionalLintParameters optionalParams, String filepath)

Repro steps

Use the package versions specified above and run a Fake target like

_Target "Lint" (fun _ ->
  let failOnIssuesFound (issuesFound: bool) =
    Assert.That(issuesFound, Is.False, "Lint issues were found")
  try
    let settings =
      Configuration.SettingsFileName
      |> Path.getFullName
      |> File.ReadAllText

    let lintConfig =
      FSharpLint.Application.ConfigurationManagement.loadConfigurationFile settings
    let options =
      { Lint.OptionalLintParameters.Default with Configuration = Some lintConfig }

    !!"**/*.fsproj"
    |> Seq.collect (fun n -> !!(Path.GetDirectoryName n @@ "*.fs"))
    |> Seq.distinct
    |> Seq.map (fun f ->
         match Lint.lintFile options f with
         | Lint.LintResult.Failure x -> failwithf "%A" x
         | Lint.LintResult.Success w ->
           w
           |> Seq.filter (fun x ->
                match x.Fix with
                | None -> false
                | Some fix -> fix.FromText <> "AltCover_Fake")) // special case
    |> Seq.concat
    |> Seq.fold (fun _ x ->
         printfn "Info: %A\r\n Range: %A\r\n Fix: %A\r\n====" x.Info x.Range x.Fix
         true) false
    |> failOnIssuesFound
  with ex ->
    printfn "%A" ex
    reraise())

Expected behavior

Linting happens

Actual behavior

Exception thrown as above

Known workarounds

Back off to previous tool versions.

Related information

• Operating system - Windows
• Branch - NuGet released package
• .NET Runtime, CoreCLR or Mono Version - .net core 3.1.101

[SteveGilham]

More digging – root cause is that the compiler services assembly doesn’t do semantic versioning properly, if at all.

Decompiling the v34.1 compiler services assembly shows that whether by design or oversight there’s a parameter not marked as [OptionalArgument], though the .fsi file check in that added the 2 new arguments marks all arguments with a leading ? –

public static FSharpChecker Create([OptionalArgument] FSharpOption<int> projectCacheSize,
 [OptionalArgument] FSharpOption<bool> keepAssemblyContents,
 [OptionalArgument] FSharpOption<bool> keepAllBackgroundResolutions,
 [OptionalArgument] FSharpOption<ReferenceResolver.Resolver> legacyReferenceResolver,
 [OptionalArgument] FSharpOption<FSharpFunc<Tuple<string, DateTime>,
 FSharpOption<Tuple<object, IntPtr, int>>>> tryGetMetadataSnapshot,
 [OptionalArgument] FSharpOption<bool> suggestNamesForErrors,
 [OptionalArgument] FSharpOption<bool> keepAllBackgroundSymbolUses, 
[OptionalArgument] FSharpOption<bool> enableBackgroundItemKeyStoreAndSemanticClassification)

while decompiling the Create() call in FSharpLint 0.13.0 shows it’s expanded to the missing method signature with fewer trailing arguments

	FSharpChecker checker = FSharpChecker.Create((FSharpOption<int>)null,
 (FSharpOption<bool>)null,
 (FSharpOption<bool>)null,
 (FSharpOption<Resolver>)null,
 (FSharpOption<FSharpFunc<Tuple<string, DateTime>, FSharpOption<Tuple<object, IntPtr,
 int>>>>)null,
 (FSharpOption<bool>)null);
``

[SteveGilham]

This issue no longer reproduces with 0.13.2, now that I’ve tracked the FSharpLint API change between 0.13.1 and 0.13.2.