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Issue #18451 showed that we could get an infinite type, through
over-use of occCheckExpand in the kind of an /occurrence/ of a
type variable.
See Note [Occurrence checking: look inside kinds] in GHC.Core.Type
This patch fixes the problem by making occCheckExpand less eager
to expand synonyms in kinds.
It also improves pretty printing of kinds, by *not* suppressing
the kind on a tyvar-binder like
(a :: Const Type b)
where type Const p q = p. Even though the kind of 'a' is Type,
we don't want to suppress the kind ascription. Example: the
error message for polykinds/T18451{a,b}. See GHC.Core.TyCo.Ppr
Note [Suppressing * kinds].
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