Fix #1292: give position when failing to emit switch on annotated match

This fix gives the position and refactors the code that gave off
warnings, but it also begs the question - should we be able to handle
emitting a switch for the following case:

```scala
(x: @switch) match {
  case 'a' if somePredicate(x) => // ...
  case 'b' => // ...
}
```

Currently if there is a guard, the patternmatcher will fail to emit
a switch. Scalac can handle these cases currently.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/dotty/tools/dotc/transform/PatternMatcher.scala b/src/dotty/tools/dotc/transform/PatternMatcher.scala
index d77dcaae4..fd89696a8 100644
--- a/src/dotty/tools/dotc/transform/PatternMatcher.scala
+++ b/src/dotty/tools/dotc/transform/PatternMatcher.scala
@@ -434,7 +434,7 @@ class PatternMatcher extends MiniPhaseTransform with DenotTransformer {thisTrans
         }
       } else {
         if (dealiased hasAnnotation defn.SwitchAnnot)
-          ctx.warning("failed to emit switch for `@switch` annotated match")
+          ctx.warning("failed to emit switch for `@switch` annotated match", scrut.pos)
         None
       }
     }