add regression test for #11145.

The original TH failure observed by the ticket, namely that Template
Haskell quotes of data instance GADTs are broken, is not observable
anymore in HEAD. I therefore just added the corresponding regression
test.

Test Plan: ./validate

Reviewers: goldfire, austin, thomie, jstolarek, bgamari

Reviewed By: bgamari

Differential Revision: https://phabricator.haskell.org/D1978

GHC Trac Issues: #11145

testsuite/tests/th/T11145.hs

+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TemplateHaskell #-}
+module T11145 where
+
+data family Fuggle x y
+
+[d| data instance Fuggle Int (Maybe (a,b)) where
+      MkFuggle :: Fuggle Int (Maybe Bool) |]

testsuite/tests/th/T11145.stderr

+
+T11145.hs:8:1: error:
+    • Data constructor ‘MkFuggle’ returns type ‘Fuggle
+                                                  Int (Maybe Bool)’
+        instead of an instance of its parent type ‘Fuggle
+                                                     Int (Maybe (a_a4MU, b_a4MV))’
+    • In the definition of data constructor ‘MkFuggle’
+      In the data instance declaration for ‘Fuggle’

testsuite/tests/th/all.T

@@ -396,3 +396,4 @@ test('TH_finalizer', normal, compile, ['-v0'])
 test('T10603', normal, compile, ['-ddump-splices -dsuppress-uniques'])
 test('T11452', normal, compile_fail, ['-v0'])
 test('T9022', normal, compile_and_run, ['-v0'])
+test('T11145', normal, compile_fail, ['-v0'])
\ No newline at end of file