Match expression inside match expression leads to offside error.

[nojaf]

I believe I found a regression when it comes to the offside warnings.

Repro steps

When getting the untyped AST from the following example:

let u = ""

match!
    match! u with
    | null -> ""
    | s -> s
    with
| "" -> x
| _ -> failwith ""

An offside error will be reported. [|tmp.fsx (9,1)-(9,2) parse warning Possible incorrect indentation: this token is offside of context started at position (5:5). Try indenting this token further or using standard formatting conventions.|]

This does not occur when the match expression is multiline but without a match expression.

let u = ""

match!
    someLongFunctionApplication
        arg1
        arg2
        arg3
    with
| "" -> x
| _ -> failwith ""

Expected behavior

In FCS 39 no warnings were produced.

Actual behavior

Offside errors only in the cases where the match expression is a match (bang) expression.

Known workarounds

The indentation of the original code could be changed to get around this warning:

let u = ""

match!
    match! u with
    | null -> ""
    | s -> s
with
| "" -> x
| _ -> failwith ""

This is also valid but not in the current version of F#.

Related information

Provide any related information (optional):

FSharp.Compiler.Service 40.0.1-preview.21418.3

I suspect the recent changes of @Happypig375 might be related here.
//cc @dsyme

[dsyme]

Thanks for the report!

[dsyme]

Note I'm only seeing this on main with /langversion:preview

Strictly speaking this doesn't make it a regression but rather a bug in the preview feature

@Happypig375 Could you take a look?

C:\GitHub\dsyme\fsharp>fsc b.fs --parseonly
Microsoft (R) F# Compiler version 11.4.2.0 for F# 5.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

warning FS0075: The command-line option '--parseonly' is for test purposes only

C:\GitHub\dsyme\fsharp>artifacts\bin\fsc\Debug\net472\fsc.exe b.fs --parseonly
Microsoft (R) F# Compiler version 11.3.3.0 for F# 5.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

warning FS0075: The command-line option '--parseonly' is for test purposes only

C:\GitHub\dsyme\fsharp>artifacts\bin\fsc\Debug\net472\fsc.exe b.fs --parseonly --langversion:preview
Microsoft (R) F# Compiler version 11.3.3.0 for F# 5.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

warning FS0075: The command-line option '--parseonly' is for test purposes only

b.fs(8,1): warning FS0058: Possible incorrect indentation: this token is offside of context started at position (4:5). Try indenting this token further or using standard formatting conventions.

[nojaf]

Hmm, I got this when parsing text via dotnet fsi.

My script:

#i "nuget:https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-tools/nuget/v3/index.json"
#r "nuget:FSharp.Compiler.Service, 40.0.1-preview.21418.3"

open FSharp.Compiler.Text
open FSharp.Compiler.CodeAnalysis

let fileName = "tmp.fsi"
let checker = FSharpChecker.Create()

let parsingOptions =
    { FSharpParsingOptions.Default with
          SourceFiles = [| fileName |] }

let getAst fileName source =
    async {
        let! tree = checker.ParseFile(fileName, source |> SourceText.ofString, parsingOptions)
        if tree.Diagnostics.Length > 0 then printfn "%A" tree.Diagnostics
        return tree.ParseTree
    }
    |> Async.RunSynchronously

getAst
    "tmp.fsx"
    """
let u = ""

match!
    match! u with
    | null -> ""
    | s -> s
    with
| "" -> x
| _ -> failwith ""
    """

Run with local fsi:

dotnet artifacts\bin\fsi\Debug\net5.0\fsi.dll "folder\prerelease-fcs.fsx"

[Happypig375]

My F# compiler setup has been wonky ever after the requirement of .NET 6 builds. My Visual Studio 2019 won't show IntelliSense, builds fail, etc. I would need to fix it before looking into this issue.

[auduchinok]

@Happypig375 JFYI it's now possible to use .NET 5 again, if you open service/FSharp.Compiler.Service.sln solution instead.

[vzarytovskii]

Strictly speaking this doesn't make it a regression but rather a bug in the preview feature

It is part of langaugeVersion60 in dev17.0.
Looking at it. It's due to changes when we calculate offset for | _, CtxtMatch offsidePos :: _ when feature is enabled I think.

[dsyme]

@Happypig375 Have you had a chance to look at this? Thanks

[Happypig375]

Nope, been busy, sorry

[dsyme]

@Happypig375 OK.

Since this is a regression in F# 6.0 it's important to get to the bottom of it :) It would be really helpful if you can help us with that.

[DalekBaldwin]

I'm seeing what is probably the same underlying issue, but in try/with expressions, while upgrading an existing project to F# 6. That is, code that used to compile fine with the with indented now only compiles with the with unindented back to the same position as the corresponding try token.