Fix #16030 by refactoring IfaceSyn's treatment of GADT constructors

Summary:
GHCi's `:info` command was pretty-printined GADT
constructors suboptimally in the following ways:

1. Sometimes, fields were parenthesized when they did not need it,
   e.g.,

```lang=haskell
data Foo a where
  MkFoo :: (Maybe a) -> Foo a
```

   I fixed this by refactoring some code in `pprIfaceConDecl` to be a
   little smarter with respect to GADT syntax. See `pprFieldArgTy`
   and `pprArgTy`.
2. With `-fprint-explicit-kinds` enabled, there would be times when
   specified arguments would be printed without a leading `@` in GADT
   return types, e.g.,

```lang=haskell
data Bar @k (a :: k) where
  MkBar :: Bar k a
```

   It turns out that `ppr_tc_app`, the function which pretty-prints
   these return types, was not using the proper machinery to print
   out the arguments, which caused the visibilities to be forgotten
   entirely. I refactored `ppr_tc_app` to do this correctly.

Test Plan: make test TEST=T16030

Reviewers: goldfire, bgamari, simonpj

Reviewed By: simonpj

Subscribers: simonpj, rwbarton, carter

GHC Trac Issues: #16030

Differential Revision: https://phabricator.haskell.org/D5440

1 files changed, 1 insertions, 1 deletions

diff --git a/compiler/iface/IfaceType.hs b/compiler/iface/IfaceType.hs
index c92c5d00f5..ebbc68755b 100644
--- a/compiler/iface/IfaceType.hs
+++ b/compiler/iface/IfaceType.hs
@@ -24,7 +24,7 @@ module IfaceType (
         IfaceForAllBndr, ArgFlag(..), ShowForAllFlag(..),
         mkIfaceForAllTvBndr,
 
-        ifForAllBndrVar, ifForAllBndrName,
+        ifForAllBndrVar, ifForAllBndrName, ifaceBndrName,
         ifTyConBinderVar, ifTyConBinderName,
 
         -- Equality testing