Add a public API to determine if a switch expression was valid in C# 6.

[gafter]

Fixes #13334

@dotnet/roslyn-compiler Please review this small compiler change (adding an API)
@jaredpar @AnthonyDGreen Please review the API change itself. Is this the right place for it?

[mattwar]

Why does this need to be an API that will appear in the completion list of all users of ITypeSymbol? This seems like a well known fact that could be in a private helper that the edit-and-continue logic uses.

[gafter]

It doesn't "need" to be in any particular place, but this is a language question implemented in the compiler layer. Generally speaking, when language questions need to be asked by other layers, they should do so via public APIs. I think you're suggesting that perhaps this language question is not generally useful enough to expose publicly?

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In reply to: 77065023 [](ancestors = 77065023)

[tmat]

Would this be more appropriate to expose on some more specialized type, perhaps on SemanticModel? Also it would be nice to avoid V6 in the name. Perhaps IsPrimitiveSwitchType?

[gafter]

I don't think there is a simpler name. For example, it includes enum types.

The implementation is simple enough that I think it can just be coped into the E&C code. I'll place the implementation in the issue for this, and close this PR.

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In reply to: 77084316 [](ancestors = 77084316)

[jcouv]

Maybe I'm missing something, but it looks like the EnC project does not have access to compiler internals (and trying to give it access causes many conflicts). I'm assuming there is a reason for that.
If so, having this public compiler API will help.

[mattwar]

EnC doesn't need access to compiler internals to copy the logic that is inside that API.