Fix #10747: Raise type error on unreducible match type

[OlivierBlanvillain]

The second commit is needed to that we don't unnecessarily try to reduce a match type right-hand side when that's not required. One example where thing go wrong otherwise is tuple's Elem type:

/** Type of the element at position N in the tuple X */
type Elem[X <: Tuple, N <: Int] = X match {
  case x *: xs =>
    N match {
      case 0 => x
      case S[n1] => Elem[xs, n1]
    }
}

Given a concrete instantiation of X, trying to reduce the right-hand side of this type results in a type error because Elem[Unit, n1] is a match error. Obviously, we could circumvent that issue by adding a case _ => Nothing, but getting a type error leads to better type safety. Furthermore, it matches the match expression "execution model" where a right-hand side is only executed if the overall expression reduces to that case.

[johnynek]

note that: #12974 might be a good example of a test to add. I hope this PR could fix that issue as well.

[odersky]

Nice work! Sorry for taking so long to review.

[odersky]

Needs a rebase.

[OlivierBlanvillain]

Rebased. Given that this PR changes the semantics of match types, should we still merge it directly on master or should it go to another branch?

[smarter]

I'd say either we should make an exception and get this in 3.1.0-RC2 or wait for 3.2

[johnynek]

I'm really happy this has been fixed. I hope it goes in sooner than later to minimize the amount of broken code that might be written before the fix is out.

Currently there is very little code in the wild using match types. The longer we wait the less true that is.

[smarter]

Assigned to @anatoliykmetyuk for merging and backporting.

[dwijnand]

#13519 is the backport.