Roslyn: Probably a bug with CS8302

[Youssef1313]

Steps to Reproduce:

Use #error version:7.1 with LangVersion set to 7.1

Expected Behavior:

No diagnostic

Actual Behavior:

error CS8302: Feature 'version' is not available in C# 7.1. Please use language version 7.1 or greater.

The error message looks very strange.

The problem lies in DirectiveParser.cs, more specifically, this part of code:

if (this.Options.LanguageVersion != LanguageVersion.Preview &&
    errorText.StartsWith(versionMarker, StringComparison.Ordinal) &&
    LanguageVersionFacts.TryParse(errorText.Substring(versionMarker.Length), out var languageVersion))
{
    ErrorCode error = this.Options.LanguageVersion.GetErrorCode();
    eod = this.AddError(eod, triviaOffset, triviaWidth, error, "version", new CSharpRequiredLanguageVersion(languageVersion));
}

I think the if condition needs to compare the parsed languageVersion with this.Options.LanguageVersion or this.Options.SpecifiedLanguageVersion (not sure what's the difference between them, hope someone can clear this up for me).

[RikkiGibson]

It's not clear to me what the purpose of #error version:7.1 is. It seems to cause us to give a LangVersion diagnostic, using the indicated language version as the required language version. Do you know what this is for @jcouv?

It's a little bit funny in its behavior, for example in a C# 6 project with #error version:5 I get: Feature 'version' is not available in C# 6. Please use language version 5 or greater.

As far as I know #error version is not a documented or supported feature, it is just a mechanism we use from time to time to try and determine what compiler is being used when compiling in some indirect way.

[Youssef1313]

It's not clear to me what the purpose of #error version:7.1 is. It seems to cause us to give a LangVersion diagnostic, using the indicated language version as the required language version. Do you know what this is for @jcouv?

@RikkiGibson It's unclear to me what's the purpose of it. But the error message looks weird.

As far as I know #error version is not a documented or supported feature, it is just a mechanism we use from time to time to try and determine what compiler is being used when compiling in some indirect way.

Whoops, I recently added a PR in dotnet/docs adding information about #error version. Should I revert it?

[RikkiGibson]

Whoops, I recently added a PR in dotnet/docs adding information about #error version. Should I revert it?

I think it's fine.

[jcouv]

Should I revert it?

I think it's okay to document #error version as it has been very useful for troubleshooting, but I would not document #error version:<version> as this is mostly useful to the compiler team.

[jcouv]

Since #error version:<version> is not meant for users, I don't worry about the quality of the diagnostic message. I'll go ahead and close this issue as by-design. Thanks