Issue with consuming DLLs built with .NET7 from .NET6

[vzarytovskii]

1. Build a library which has langversion=7 and targets net6.0, using net7.0 SDK
   1. Library should involve some sort of SRTP code (with either new or old syntax).
2. Reference this library in a project which targets net6.0 and uses net6.0 SDK

This will result in Error reading/writing metadata for the F# compiled DLL '%s'. Was the DLL compiled with an earlier version of the F# compiler? (error: 'u_trait_sln')..

This is sort of by design, since we're technically picking some data which is unknown to the old compiler.

The general recommendation is to move to .NET7 SDK on the consumer side, or target .NET7, when langversion 7 is needed to be used.

[dsyme]

Do we have a repro?

We should adjust /langversion:6.0 so that it never emits the new staticTyOpt.

[vzarytovskii]

I think /langversion:6 doesn't emit it, only in 7, problem is with consuming the library with langversion=7 from 6.0 sdk

When switching library project to langversion=7, the issue goes away on the consuming side.

[vzarytovskii]

@dsyme one thing we can do is to warn that it can be technically unsupported.
When the language version is 7, and tfm is <7.0

[wallymathieu]

I've tried locally to reproduce the issue. Simply by building the project on the branch v1.3
https://github.com/fsprojects/FSharpPlus/tree/v1.3
then copy the resulting net6 dll into a folder with a net6 global json

{
  "sdk": {
    "version": "6.0.100",
    "rollForward": "latestFeature"
  }
}

repro.fsx

#r "FSharpPlus.dll"
open FSharpPlus;;

It seems that even with LangVersion 6 specified I can reproduce the issue.