SI-8546 Pattern matcher analysis foiled by over-widening

In the enclosed test, the prefix checkable type
`ModuleTypeRef(F2.this, C)` was being inadvertently widened to
`ModuleTypeRef(F2[?], C)`. This started after some misguided
future-proofing in SI-6771 / 3009916.

This commit changes the `dealiasWiden` to a `delias`.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala b/src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala
index 2893cbdf45..894f959319 100644
--- a/src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala
+++ b/src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala
@@ -149,14 +149,15 @@ trait TreeAndTypeAnalysis extends Debugging {
       object typeArgsToWildcardsExceptArray extends TypeMap {
         // SI-6771 dealias would be enough today, but future proofing with the dealiasWiden.
         // See neg/t6771b.scala for elaboration
-        def apply(tp: Type): Type = tp.dealiasWiden match {
+        def apply(tp: Type): Type = tp.dealias match {
           case TypeRef(pre, sym, args) if args.nonEmpty && (sym ne ArrayClass) =>
             TypeRef(pre, sym, args map (_ => WildcardType))
           case _ =>
             mapOver(tp)
         }
       }
-      debug.patmatResult(s"checkableType($tp)")(typeArgsToWildcardsExceptArray(tp))
+      val result = typeArgsToWildcardsExceptArray(tp)
+      debug.patmatResult(s"checkableType($tp)")(result)
     }
 
     // a type is "uncheckable" (for exhaustivity) if we don't statically know its subtypes (i.e., it's unsealed)