Pattern synonyms and higher rank types
This patch fixes two separate bugs which contributed to making Trac #13752 go wrong 1. We need to use tcSubType, not tcUnify, in tcCheckPatSynDecl.tc_arg. Reason: Note [Pattern synonyms and higher rank types] 2. TcUnify.tc_sub_type had a special case designed to improve error messages; see Note [Don't skolemise unnecessarily]. But the special case was too liberal, and ended up using unification (which led to rejecting the program) when it should instead taken the normal path (which accepts the program). I fixed this by making the test more conservative.
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