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`substituteSymbols` is not sophisticated enough to
operate on `TypeSkolem`-s which are based on one of the
"from" symbols.
The pertinant usage of `substituteSymbols` for this bug in
in `Extender`. Recapping on that transform:
// orig
class C[T](...) extends AnyVal { def foo[U] = <rhs> }
// transform
class C[T] extends AnyVal { ... }
object C { def foo$extension[T', U'] = <rhs'> }
Where `<rhs'>` has been subtituted with, among other things,
`[T, U] ~> [T', U']`.
In this case our expected type contains a new type parameter
(of the extension method), whereas the type of the RHS contains
an existential skolem still pinned to the corresponding class type
parameter.
tree.tpe = Observable1#7037[_$1#12344]
<_$1#12344>.info = <: T#7040
pt = Observable1#7037[T#15644]
The limitation of substution is lamented in the comments
of `adaptMismatchedSkolems`, which faces the harder version of
the issue where the skolems are in the expected type.
But, we're in the "easy" case with the skolems in the tree's type;
we can cast our way out of the problem.
See also f335e447 / ed915c54.
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