No support for 'preview' language versions?

[FiniteReality]

I'm writing some code to try out the new 'unmanaged constructed types' and 'static local functions' features, and as expected OmniSharp is flagging this up with error CS8652, stating that the feature is not supported and currently in preview.

However, the error says to use the preview language version to use these features, and upon specifying this in the LangVersion property, these errors remain. To hide these errors, I had to specify 8.0 as the value, instead of what the error had specified.

I believe this is due to this switch not including preview as a possible value, though I may be wrong since I'm not familiar with the code:

omnisharp-roslyn/src/OmniSharp.MSBuild/ProjectFile/PropertyConverter.cs

Lines 38 to 53 in 46c814a

	switch (propertyValue.ToLower())
	{
	case "latest": return LanguageVersion.Latest;
	case "iso-1": return LanguageVersion.CSharp1;
	case "iso-2": return LanguageVersion.CSharp2;
	case "3": return LanguageVersion.CSharp3;
	case "4": return LanguageVersion.CSharp4;
	case "5": return LanguageVersion.CSharp5;
	case "6": return LanguageVersion.CSharp6;
	case "7": return LanguageVersion.CSharp7;
	case "7.1": return LanguageVersion.CSharp7_1;
	case "7.2": return LanguageVersion.CSharp7_2;
	case "7.3": return LanguageVersion.CSharp7_3;
	case "8.0": return LanguageVersion.CSharp8;
	default: return LanguageVersion.Default;
	}

[FiniteReality]

I don't think there'll be any breaking changes if that entire snippet was replaced with LanguageVersionFacts.TryParse, so that could be a possible fix ensuring that this won't happen again in the future.

[filipw]

You are right, the Preview value is missing (as well as another new value LatestMajor) - both came in Roslyn 3.0 and we managed to miss them, thanks for opening the issue.

The reason why the built-in Roslyn language version parser was not used in the first place is because when the project parsing code was originally added, the LanguageVersion.TryParse (or its old version LanguageVersionFacts.TryParse) API didn't exist yet - it only came in Roslyn 2.1.

[FiniteReality]

Ah, I had guessed as much. I'm alright to make a PR to fix this then, right? I'll probably replace it with the TryParse method I mentioned, which is indeed under LanguageVersionFacts, I had missed the class declaration a few lines above it while browsing the code.

[filipw]

sure, please do!