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 higher-rank 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 higher-rank) `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.

1 files changed, 6 insertions, 6 deletions

diff --git a/testsuite/tests/ghci.debugger/scripts/print027.stdout b/testsuite/tests/ghci.debugger/scripts/print027.stdout
index eb5b363693..3117eace87 100644
--- a/testsuite/tests/ghci.debugger/scripts/print027.stdout
+++ b/testsuite/tests/ghci.debugger/scripts/print027.stdout
@@ -1,6 +1,6 @@
-+ = (_t1::t1)
-print = (_t2::t1)
-log = (_t3::t1)
-head = (_t4::[a] -> a)
-tail = (_t5::[a1] -> [a1])
-fst = (_t6::(a2, b) -> a2)
++ = (_t1::Num a => a -> a -> a)
+print = (_t2::Show a1 => a1 -> IO ())
+log = (_t3::Floating a2 => a2 -> a2)
+head = (_t4::[a4] -> a4)
+tail = (_t5::[a7] -> [a7])
+fst = (_t6::(a11, b) -> a11)