Stringinterpolation requires langVersion in .NET 5

Describe the bug If I want to use string interpolation from F# 5 in a new .NET 5 project, I get an error in the editor that tells me, that string interpolation is not available in F# 4.7. As far as I know, F# 5 is now the default language version and dotnet build runs successfully without this error.

To Reproduce Steps to reproduce the behaviour:

1. Use string interpolation in an net5.0 project without LangVersion

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net5.0</TargetFramework>
    <RootNamespace>test</RootNamespace>
  </PropertyGroup>

let test = "test"
$"%s{test}"

Expected behaviour I would expect that the language version is the same for editor and compiler.

Screenshots

Environment (please complete the following information):

• OS: Windows (but inside of a mcr.microsoft.com/dotnet/sdk:5.0-focal dev container)
• Ionide version: 4.17.0
• VSCode version: Version: 1.51.0 (user setup)
• dotnet SDK version: 5.0.100
• mono / .Net Framework version: -

Additional context I created a reproduction repo here: https://github.com/NicoVIII/repro-bug

I don’t rule out that I do something wrong. If this is the case: Sorry!

[baronfel]

We’re actively working on this, you can follow the status here: https://github.com/fsharp/FsAutoComplete/pull/656

[Krzysztof-Cieslak]

This is fixed in Ionide 5.0 preview