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Simon Peyton Jones authored
The pre-subsuption and pre-matching functions should NEVER make bogus bindings of type variables, although they are free to bale out and make too few bindings. I hadn't been thiking carefully enough about this, and there were two separate bugs. - Firstly, in pre-subsumption we must ignore the 'theta' part of any overloaded type. - Second, in pre-matching, we must return the empty subustition on a mis-match, rather than returning the substitution so far. This bug showed up when compiling Data.Generics.Schemes.hs, and is imortalised in test tc206
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