[SILGen] Silence redundant unreachable enum case warning

[theblixguy]

When an enum case is already handled, for example:

enum Foo { case a, b }
func takesFoo(arg: Foo) {
  switch arg {
    case _: break
    case .b: break
  }
}

We would get two warnings on case .b: case is already handled by previous patterns; consider removing it & case will never be executed. The first warning comes from Space Engine and the second warning comes from SILGenPattern.

So, silence the (redundant) warning emitted by SILGenPattern for switch statements (except default case).

Resolves #54657
Resolves rdar://problem/59655513

[jckarter]

IIRC there isn't perfect overlap between the two diagnostics, so disabling either one will cause us to miss cases the other doesn't catch either. @CodaFi how does this sound to you now?

[CodaFi]

The space engine only has incomplete redundancy checking for expression patterns - there we do something awful that approximates redundancy checking for (non-range) literals. Proper patterns always emit overlap diagnostics, so we could at least ditch SIL's diagnostics for those cases.

[theblixguy]

I have updated the implementation to check for ExprPatterns and continue to emit the diagnostic in that case.

Is there a test case that triggers redundant warning for ExprPattern today? I tried a few test cases but it doesn’t seem to trigger redundant diagnostics.

[theblixguy]

Ping. Does the change look good now @CodaFi?

[theblixguy]

Ping.

[CodaFi]

LGTM. Joe can sign off on SIL.

[jckarter]

Looks good to me too.

[theblixguy]

@swift-ci please smoke test