patmat selector gets widened too much

[smarter]

object Test {
  type T <: AnyRef

  def test(x: T): Unit = {
    x match {
      case foo =>
        val z: T = foo
    }
  }
}

This compiles with scalac but fails in dotty with:

-- [E007] Type Mismatch Error: try/collect.scala -------------------------------
7 |        val z: T = foo
  |                   ^^^
  |                   found:    Object(foo)
  |                   required: Test.T

This happens because we aggressively widen the selector in typedMatch:

        val selType = widenForMatchSelector(
            fullyDefinedType(sel1.tpe, "pattern selector", tree.pos))

Replacing widenForMatchSelector for by a call to .widen fixes the issue, but breaks one test: tests/patmat/for.scala which is run with -Ycheck-all-patmat and should not report any exhaustivity warning, I'm not sure why there's a difference here. A possible solution would be to call widenForMatchSelector inside the exhaustivity checker, but I don't know if that gives the correct semantics.

[liufengyun]

Closely related #1330 , related #1710

[liufengyun]

The following change also solves the problem, I'm not sure if it's the correct way to do it.

   def widenForMatchSelector(tp: Type)(implicit ctx: Context): Type = tp.widen match {
-    case tp: TypeRef if !tp.symbol.isClass =>
+    case tp: TypeRef if !tp.symbol.is(Deferred) =>
         widenForMatchSelector(tp.superType)
      case tp: HKApply =>
        widenForMatchSelector(tp.superType)
      case tp: AnnotatedType =>
        tp.derivedAnnotatedType(widenForMatchSelector(tp.tpe), tp.annot)
      case tp => tp
    }

[odersky]

I can't remember why we introduced widenForMatchSelector. I'd replace with widen and maybe Fengyun can have a look at the exhaustivity problem in for.scala?

[liufengyun]

OK, I'll replace it just with widen and fix the problem with exhaustivity check.