Fix embarrassing infinite loop in pprParendType

Summary:
`pprParendType` was missing an explicit case for
`EqualityT`, which caused it to fall through to a catch-all case
that invokes `ppr`. But `ppr` itself does not have a case for a
partial application of `EqualityT`, so //it// falls back to
`pprParendType`, resulting in an infinite loop!

The fix is simple: add a case for `EqualityT` in `pprParendType`.
While I was in the neighborhood, I removed the catch-call case in
`pprParendType` to make this sort of mistake less likely to happen
in the future.

Test Plan: make test TEST=T15985

Reviewers: bgamari, monoidal, simonpj

Reviewed By: monoidal, simonpj

Subscribers: rwbarton, carter

GHC Trac Issues: #15985

Differential Revision: https://phabricator.haskell.org/D5403

1 files changed, 3 insertions, 1 deletions

diff --git a/libraries/template-haskell/Language/Haskell/TH/Ppr.hs b/libraries/template-haskell/Language/Haskell/TH/Ppr.hs
index 138cf62b24..621c0f5fcc 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Ppr.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Ppr.hs
@@ -739,7 +739,9 @@ pprParendType tuple | (TupleT n, args) <- split tuple
                     , length args == n
                     = parens (commaSep args)
 pprParendType (ImplicitParamT n t)= text ('?':n) <+> text "::" <+> ppr t
-pprParendType other               = parens (ppr other)
+pprParendType EqualityT           = text "(~)"
+pprParendType t@(ForallT {})      = parens (ppr t)
+pprParendType t@(AppT {})         = parens (ppr t)
 
 pprUInfixT :: Type -> Doc
 pprUInfixT (UInfixT x n y) = pprUInfixT x <+> pprName' Infix n <+> pprUInfixT y