Set `ImpredicativeTypes` during :print command. (#14828)
If ImpredicativeTypes is not enabled, then `:print <term>` will fail if the type of <term> has nested `forall`s or `=>`s. This is because the GHCi debugger's internals will attempt to unify a metavariable with the type of <term> and then display the result, but if the type has nested `forall`s or `=>`s, then unification will fail. As a result, `:print` will bail out and the unhelpful result will be `<term> = (_t1::t1)` (where `t1` is a metavariable). Beware: <term> can have nested `forall`s even if its definition doesn't use RankNTypes! Here is an example from #14828: class Functor f where fmap :: (a > b) > f a > f b Somewhat surprisingly, `:print fmap` considers the type of fmap to have nested foralls. This is because the GHCi debugger sees the type `fmap :: forall f. Functor f => forall a b. (a > b) > f a > f b`. We could envision deeply instantiating this type to get the type `forall f a b. Functor f => (a > b) > f a > f b`, but this trick wouldn't work for higherrank types. Instead, we adopt a simpler fix: enable `ImpredicativeTypes` when using `:print` and friends in the GHCi debugger. This is allows metavariables to unify with types that have nested (or higherrank) `forall`s/`=>`s, which makes `:print fmap` display as `fmap = (_t1::forall a b. Functor f => (a > b) > f a > f b)`, as expected. Although ImpredicativeTypes is a somewhat unpredictable from a type inference perspective, there is no danger in using it in the GHCi debugger, since all of the terms that the GHCi debugger deals with have already been typechecked.
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mentioned in merge request !2685 (closed)

mentioned in issue #12449
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