In type patterns `c match { case x: T }`, the translation would assign
the GLB of `c`'s type and `T` to the varaible `x`.

This seems to trace back to the first version of the "virtual pattern
matcher". I could not find a similar use of `glb` in that revision
of the codebase. So I'm not sure if it was a new addition, or picked
up from the previous implementation.

https://github.com/scala/scala/blob/8a9fd64129926eea35f7dca181242855f14e153f/src/compiler/scala/tools/nsc/typechecker/PatMatVirtualiser.scala#L438-L440

In the test case, the GLB collapsed to `Null` because its
computation failed (combination of f-bounds, existentials, skolems that
I don't follow), see `throw GlbFailure`. This is how it's been for 14
years (b894f80). This resulted in a cast to `Null$` failing at
runtime.

I assume GLB is fixed in Scala 3, as this core of the type system has
a new implementation. But the test case as such doesn't compile in
Scala 3 due to the non-wildcard existential.