Warn about simplifiable class constraints
Provoked by Trac #11948, this patch adds a new warning to GHC -Wsimplifiable-class-constraints It warns if you write a class constraint in a type signature that can be simplified by an existing instance declaration. Almost always this means you should simplify it right now; type inference is very fragile without it, as #11948 shows. I've put the warning as on-by-default, but I suppose that if there are howls of protest we can move it out (as happened for -Wredundant-constraints. It actually found an example of an over-complicated context in CmmNode. Quite a few tests use these weird contexts to trigger something else, so I had to suppress the warning in those. The 'haskeline' library has a few occurrences of the warning (which I think should be fixed), so I switched it off for that library in warnings.mk. The warning itself is done in TcValidity.check_class_pred. HOWEVER, when type inference fails we get a type error; and the error suppresses the (informative) warning. So as things stand, the warning only happens when it doesn't cause a problem. Not sure what to do about this, but this patch takes us forward, I think.
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