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Typechecking a pattern that defines a pattern type variable
initially assigns abstract type symbol with open type bounds.
Later on, pattern type inference kicks in to sharpen the type
of the variable based on constraints imposed by the expected
type (ie, the type of scrutinee of the pattern.)
However, before inference does this, a `TypeRef` to the abstract
type symbol can be queried for its base type with respect to some
class, which leads to it populating an internal cache. This cache
becomes stale when the underlying symbol has its type mutated.
The repercussions of this meant that a subsequent call to `baseType`
gave the wrong result (`NoType`), which lead to an `asSeenFrom`
operation to miss out of substitution of a type variable. Note the
appearance of `A` in the old type errors in the enclosed test case.
This commit takes an approach similar to 286dafbd to invalidate
caches after the mutation. I've routed both bandaids through the
same first aid kit: I'm sure over time we'll add additional calls
to this method, and additional cache invalidations within it.
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