This commit changes fixit messages from a question/suggestion to an
imperative message for protocol conformances and switch-case. Addresses
https://github.com/apple/swift/issues/67510.
Previously if the cast was unresolved, we would
emit a warning and bail with `nullptr`. This is
wrong, because the caller expects a `nullptr`
return to mean we emitted an error. Change the
diagnostic to an error to fix this. This may
appear source breaking, but in reality previously
we were failing to add the cast at all in this case,
which lead to a crash in SILGen. We really do
want to reject these cases as errors, as this
will give us a better opportunity to fall back to
type-checking as ExprPatterns, and better matches
the constraint solver type-checking.
Also while we're here, change the diagnostic for
the case where we don't have an existential context
type from the confusing "enum doesn't have member"
diagnostic to the pattern mismatch diagnostic.
rdar://107420031
The TypedPattern and IsPattern constraints were
incorrectly written, with conversions propagating
out of the patterns, when really conversions
ought to propagate into patterns. In any case, it
seems like we really want equality here. Fix the
constraints to use equality, and have the cast
constraint operate on the external pattern type
instead of the subpattern type.
