Improve typechecking of instance defaults
In an instance declaration when you don't specify the code for a method, GHC fills in from the default binding in the class. The type of the default method can legitmiately be ambiguous --- see Note [Default methods in instances] in TcInstDcls --- so typechecking it can be tricky. Trac #12220 showed that although we were dealing with that ambiguity for /vanilla/ default methods, we were not doing so for /generic/ default methods. Moreover we were dealing with it clumsily, by generating post-typechecked code. This patch fixes the bug AND deletes code! We now use the same code path for both vanilla and generic default methods; and generate /pre-typechecked/ code in both cases. The key trick is that we can use Visible Type Application to deal with the ambiguity, which wasn't possible before. Hooray. There is a small hit to performance in compiler/perf/T1969 which consists of nothing BUT instance declarations with several default methods to fill, which we now have to typecheck. The actual hit is from 724 -> 756 or 4% in that extreme example. Real world programs have vastly fewer instance decls.
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