[Haskell-cafe] Strange problem with inference

[David Kraeutmann]

~ isn't equivalent. Type signatures are solved by unification and then all
occurring constraints are solved. By putting a ~ constraint you explicitly
instruct GHC to not attempt unification.

When using associated type synonyms you need to use ~, as the choice of
TestT a depends on what instance is picked for Test a. There doesn't even
have to be an instance at all. As the unifier is iirc mostly independent
from the constraint solver, you get that error.

See the relevant paper "Associated Type Synonyms" for details.
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