Fix #6255: Don't check unreachability for Expr[T]

[liufengyun]

This seems to be the best strategy for now, given that
the implicit args trick is only used for matching Expr[T],
and case _ => is always used in such cases.

Otherwise, we will need to reason equality of Terms, which is subtle.

[smarter]

Otherwise, we will need to reason equality of Terms, which is subtle.

You can do =:= on termref which won't give any false positive, isn't that good enough?

[liufengyun]

Otherwise, we will need to reason equality of Terms, which is subtle.

You can do =:= on termref which won't give any false positive, isn't that good enough?

That will be on the safe side, but will not help in checking Expr[T], given that the parameters are not really implicit, but some trees:

      case scala.internal.quoted.Matcher.unapply[Unit](())('{1}, x$2) => 
        ()
      case scala.internal.quoted.Matcher.unapply[Unit](())('{2}, x$2) => 

[smarter]

Ah that's more complicated indeed, we do have a simple structural equality implementation for trees but I don't know if it's powerful enough for what you need: https://github.com/lampepfl/dotty/blob/eae5a38f24be24fbd6f9b583a687a7738747e5a0/compiler/src/dotty/tools/dotc/ast/TreeInfo.scala#L785-L789

[abgruszecki]

@liufengyun can we have a comment documenting why checking unreachability does not work for Expr in particular? If I understand everything correctly, it could go something like this:

// Ignore Expr for unreachability as a special case.
// Quote patterns produce repeated calls to the same unapply method,
// but with different implicit parameters.
// Since we understand repeated calls to the same unapply method to overlap
// and two unapply methods in one `match` with same arguments cannot normally have different implicits,
// the easiest solution is just to ignore Expr.

[liufengyun]

@AleksanderBG Thanks for spelling it out, yes, that is what I have in mind.

[abgruszecki]

LGTM!