"Unlikely constraint" recognition

Some constraints or combinations are relatively unlikely to appear in correct code, or at least correct code written by beginners. A few tiny examples:

(Integral a, Fractional a) =>  -- Possible, but rare, in correct code.
(Integral a, RealFloat a) =>   -- Rather less possible in correct code.
(Num a, Foldable a) =>  -- Possible, but unlikely for a beginner.
(Fractional [a]) =>
(Show (A -> B)) =>

The current error messages when constraints like these are not satisfied does nothing to suggest that they are unlikely to be correct or to explain to a beginner what kinds of mistakes can lead to errors like this. I imagine some people who teach classes using Haskell will have an idea of which specific things are worth singling out for a special explanation.

Trac metadata

Trac field	Value
Version	7.8.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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