Fix splice type variable pattern detection

[nicolasstucki]

Fixes #17039

[smarter]

• How are Select where the prefix is not a Bind handled?
• There are other cases which involve pat like case Apply(fn, pat :: Nil), should those be pat: Bind too?

[nicolasstucki]

How are Select where the prefix is not a Bind handled?

Those just get the default super.transform(tree). The splitter only needs to extract the bindings of the pattern. For types, these are the type variable inline definitions. If there is a t.Underlying that is not a pattern we assume that it is an explicit splice into the pattern. In case, t is not identified as a type variable because t is part of the path of the type.

There are other cases which involve pat like case Apply(fn, pat :: Nil), should those be pat: Bind too?

These are expression splices ${...} in the pattern. There we can have an arbitrary pattern such as ${x} or ${Expr(x)}. Unlike with types, we do not have a notation or notion of inserting an expression is a quoted pattern.

[smarter]

Do we have a spec of quoted pattern matching in the doc? I'm a bit surprised I can write t.Underlying and have t match a definition outside the pattern, I thought you'd need to use backquotes `t`.Underlying like when pattern matching on terms.