Some as-patterns could be accepted in pattern synonyms
Currently all as-patterns are rejected in pattern synonym definitions, to avoid situations like
pattern P x y <- x@(y:_)
since there's no valid reason to be able to then write something like
f [True] False = ...
this would just lead to confusion.
However, I think we could accept as-patterns where the body of the as-pattern doesn't contain any variable bindings that are accessible via the pattern synonym. In other words, this should be OK:
pattern P x <- x@(y:_)
since it's exactly equivalent to
pattern P x <- x@(_:_)
which I don't think is as confusing as the other example.
I haven't really made up my mind yet if these should be bidirectional; but they certainly could be, by just ignoring the body of the as-pattern in the wrapper; so the following two would be equivalent:
pattern P1 x = [x@(y:_)] pattern P2 x <- [x@(y:_)] where P2 x = [x]
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