Roslyn: Using declaration with ‘_’ should be a discard

[jcouv]

Sharplab

‘using var _ = ...;’ currently introduces a local in scope and it can be accessed. ‘_.ToString();’ should fail.

Tagging @chsienki

Reported on SO

[chsienki]

I thought we had another issue tracking this already, but I can't seem to find it. We've talked a few times about allowing using discards, but I don't think we ever came to a concrete conclusion.

It seems like a fairly innocuous thing to allow, but we should check with LDM first.

[jcouv]

Looking at the LDM notes, I see that we did get a decision: https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-12-03.md#discussion

Please scan those notes to check if anything else was left un-implemented.

[Joe4evr]

This was also reported on csharplang over a year ago: dotnet/csharplang#2235

[jcouv]

@chsienki I don't know what you think, but suspect that fixing this will be rather involved. Moving out to 16.9 Compiler.Nextt.