GHC Handbook

Note [WantedInv]

GHC/Tc/Utils/TcType.hs:835 compiler 

Why is WantedInv important?  Consider this implication, where
the constraint (C alpha[3]) disobeys WantedInv:

   forall[2] a. blah => (C alpha[3])
                        (forall[3] b. alpha[3] ~ b)

We can unify alpha:=b in the inner implication, because 'alpha' is
touchable; but then 'b' has escaped its scope into the outer implication.

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