Always filter generators prefixed with case.

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1365,13 +1365,10 @@ object desugar {
       }
 
       def needsNoFilter(gen: GenFrom): Boolean =
-        if (gen.checkMode == GenCheckMode.FilterAlways) // pattern was prefixed by `case`
-          isIrrefutable(gen.pat, gen.expr)
-        else (
-          gen.checkMode != GenCheckMode.FilterNow ||
-          IdPattern.unapply(gen.pat).isDefined ||
-          isIrrefutable(gen.pat, gen.expr)
-        )
+        gen.checkMode == GenCheckMode.Ignore ||
+        gen.checkMode == GenCheckMode.Check ||
+        IdPattern.unapply(gen.pat).isDefined ||
+        isIrrefutable(gen.pat, gen.expr)
 
       /** rhs.name with a pattern filter on rhs unless `pat` is irrefutable when
        *  matched against `rhs`.