Fix #12739: Handle type alias in capturing wildcards

[liufengyun]

Fix #12739: Handle type alias in capturing wildcards

[dwijnand]

[error] -- [E007] Type Mismatch Error: /__w/dotty/dotty/community-build/community-projects/shapeless/modules/deriving/src/test/scala/shapeless3/deriving/type-classes.scala:570:138 
[error] 570 |      genu.fromRepr(mkRecord[genu.MirroredElemLabels, genu.MirroredElemTypes, gent.MirroredElemLabels, gent.MirroredElemTypes](gent.toRepr(t)))
[error]     |                                                                                                                               ^^^^^^^^^^^^^^
[error]     |Found:    ?1.MirroredElemTypes
[error]     |Required: gent.MirroredElemTypes
[error]     |
[error]     |where:    ?1 is an unknown value of type deriving.Mirror.Product{
[error]     |  Kind = shapeless3.deriving.K0.type; MirroredType = T; MirroredElemTypes; 
[error]     |    MirroredElemTypes <: NonEmptyTuple
[error]     |}
[error] one error found
[error] one error found
[error] (deriving / Test / compileIncremental) Compilation failed
[error] Total time: 10 s, completed Jun 12, 2021 9:01:03 AM

[anatoliykmetyuk]

@liufengyun are you planning to get back to this one? If not, can it be closed?

[liufengyun]

@liufengyun are you planning to get back to this one? If not, can it be closed?

I investigated the community build failure some time ago, but could not figure out the cause.

I see you self-assigned, could you lend a hand, @dwijnand? Thank you.

[dwijnand]

I misread the PR, so now it's a challenge whether I can figure it out or not... :-D

[dwijnand]

Minimised:

type Foo = { type Bar }

extension (foo: Foo)
  def toBar(): foo.Bar = ???

def test(foo: Foo): foo.Bar = foo.toBar()

-- [E007] Type Mismatch Error: tests/pos/i12739-fallout.scala:6:39 -----------------------------------------------------
6 |def test(foo: Foo): foo.Bar = foo.toBar()
  |                              ^^^^^^^^^^^
  |                              Found:    ?1.Bar
  |                              Required: foo.Bar
  |
  |                              where:    ?1 is an unknown value of type Object{Bar}

@liufengyun does that ring any bells? Any ideas?

[liufengyun]

Nice minimization @dwijnand 👍

It seems dealias changes the type checking path:

      val captured = captureWildcards(wtp)
      if (captured `ne` wtp)
        return readapt(tree.cast(captured))

Previously, the if branch was not taken. After dealiasing, it's taken incorrectly.

With the minimization (which is the most difficult part), you are closer to a fix. What about you make a new PR with a proper fix and we close this one?