Fix #14167 by using isGadtSyntaxTyCon in more places

Summary:
Two places in GHC effectively attempt to //guess// whether a data type
was declared using GADT syntax:

1. When reifying a data type in Template Haskell
2. When pretty-printing a data type (e.g., via `:info` in GHCi)

But there's no need for heuristics here, since we have a 100% accurate way to
determine whether a data type was declared using GADT syntax: the
`isGadtSyntaxTyCon` function! By simply using that as the metric, we obtain
far more accurate TH reification and pretty-printing results.

This is technically a breaking change, since Template Haskell reification will
now reify some data type constructors as `(Rec)GadtC` that it didn't before,
and some data type constructors that were previously reified as `(Rec)GadtC`
will no longer be reified as such. But it's a very understandable breaking
change, since the previous behavior was simply incorrect.

Test Plan: ./validate

Reviewers: simonpj, goldfire, austin, bgamari

Reviewed By: simonpj

Subscribers: rwbarton, thomie

GHC Trac Issues: #14167

Differential Revision: https://phabricator.haskell.org/D3901

1 files changed, 2 insertions, 4 deletions

diff --git a/testsuite/tests/rename/should_fail/rnfail055.stderr b/testsuite/tests/rename/should_fail/rnfail055.stderr
index 7fc5d80bad..b9ba174519 100644
--- a/testsuite/tests/rename/should_fail/rnfail055.stderr
+++ b/testsuite/tests/rename/should_fail/rnfail055.stderr
@@ -71,10 +71,8 @@ RnFail055.hs-boot:25:1: error:
     Type constructor ‘T7’ has conflicting definitions in the module
     and its hs-boot file
     Main module: type role T7 phantom
-                 data T7 a where
-                   T7 :: a1 -> T7 a
-    Boot file:   data T7 a where
-                   T7 :: a -> T7 a
+                 data T7 a = forall a1. T7 a1
+    Boot file:   data T7 a = forall b. T7 a
     The roles do not match.
     Roles on abstract types default to ‘representational’ in boot files.
     The constructors do not match: The types for ‘T7’ differ