Prevent Pattern Match Exhaustivity Warning for `transparent`

Hi, I believe that pattern match exhaustivity warnings should not be emitted for top-level pattern match statements in transparent methods, given that the pattern match is checked and expanded at compile-time anyway.

I noticed these warnings are emitted when matching over a sealed trait/class in a transparent method body.

[Blaisorblade]

(which is not so much the case currently with implicits in Scala).

That should be addressed by @implicitNotFound (not sure Dotty implements it yet but we should). And @compileTimeOnly is indeed designed for this goal, tho it honestly seems more roundabout.

Is there a nice way currently to implement compileError? As a side note, I think that having this compileError functionality would be great because it also allows to produce nice and interpretable errors (which is not so much the case currently with implicits in Scala).

Sorry for the ambiguity, I did mean “I don’t know but we should have it one way or the other”.

[Blaisorblade]

This topic came out today on Gitter, and there’s a WIP implementation in #4964 (currently in https://github.com/lampepfl/dotty/pull/4964/commits/2710e2b9165e94d74350ff91d5c1c4f55d01b87b).