Raise parse error for `data T where`.

Empty GADTs data declarations can't be identified in type checker. This
patch adds additional checks in parser and raise a parse error when
encounter empty GADTs declarations but extension `GADTs` is not enabled.

Only empty declarations are checked in parser to avoid affecting
existing
error messages related to missing GADTs extension.

This patch should fix issue 8258.

Signed-off-by: HE, Tao <sighingnow@gmail.com>

Test Plan: make test TEST="T8258 T8258NoGADTs"

Reviewers: bgamari, mpickering, alanz, RyanGlScott

Reviewed By: bgamari, RyanGlScott

Subscribers: adamse, RyanGlScott, rwbarton, thomie, mpickering, carter

GHC Trac Issues: #8258

Differential Revision: https://phabricator.haskell.org/D4350

7 files changed, 17 insertions, 2 deletions

diff --git a/testsuite/tests/parser/should_compile/T8258.hs b/testsuite/tests/parser/should_compile/T8258.hs
new file mode 100644
index 0000000000..18d6483973
--- /dev/null
+++ b/testsuite/tests/parser/should_compile/T8258.hs
@@ -0,0 +1,5 @@
+{-# LANGUAGE GADTs #-}
+
+module T8258 where
+
+data T where
diff --git a/testsuite/tests/parser/should_compile/all.T b/testsuite/tests/parser/should_compile/all.T
index cc9771087f..1ca6d7ebad 100644
--- a/testsuite/tests/parser/should_compile/all.T
+++ b/testsuite/tests/parser/should_compile/all.T
@@ -100,6 +100,7 @@ test('T7118', normal, compile, [''])
 test('T7776', normal, compile, [''])
 test('RdrNoStaticPointers01', [], compile, [''])
 test('T5682', normal, compile, [''])
+test('T8258', normal, compile, [''])
 test('T9723a', normal, compile, [''])
 test('T9723b', normal, compile, [''])
 test('T10188', normal, compile, [''])
diff --git a/testsuite/tests/parser/should_fail/T8258NoGADTs.hs b/testsuite/tests/parser/should_fail/T8258NoGADTs.hs
new file mode 100644
index 0000000000..1080233bcd
--- /dev/null
+++ b/testsuite/tests/parser/should_fail/T8258NoGADTs.hs
@@ -0,0 +1,3 @@
+module T8258NoGADTs where
+
+data T where
diff --git a/testsuite/tests/parser/should_fail/T8258NoGADTs.stderr b/testsuite/tests/parser/should_fail/T8258NoGADTs.stderr
new file mode 100644
index 0000000000..35f5306274
--- /dev/null
+++ b/testsuite/tests/parser/should_fail/T8258NoGADTs.stderr
@@ -0,0 +1,5 @@
+
+T8258NoGADTs.hs:3:8: error:
+    Illegal keyword 'where' in data declaration
+    Perhaps you intended to use GADTs or a similar language
+    extension to enable syntax: data T where
diff --git a/testsuite/tests/parser/should_fail/all.T b/testsuite/tests/parser/should_fail/all.T
index ef47ed3394..2cb9c49de2 100644
--- a/testsuite/tests/parser/should_fail/all.T
+++ b/testsuite/tests/parser/should_fail/all.T
@@ -87,6 +87,7 @@ test('T5425', normal, compile_fail, [''])
 test('T984', normal, compile_fail, [''])
 test('T7848', normal, compile_fail, ['-dppr-user-length=100'])
 test('ExportCommaComma', normal, compile_fail, [''])
+test('T8258NoGADTs', normal, compile_fail, [''])
 test('T8430', literate, compile_fail, [''])
 test('T8431', compile_timeout_multiplier(0.05),
      compile_fail, ['-XAlternativeLayoutRule'])
diff --git a/testsuite/tests/polykinds/T11640.hs b/testsuite/tests/polykinds/T11640.hs
index 16d9f7ccff..bbb4a53bfc 100644
--- a/testsuite/tests/polykinds/T11640.hs
+++ b/testsuite/tests/polykinds/T11640.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE RankNTypes, TypeInType #-}
+{-# LANGUAGE GADTs, RankNTypes, TypeInType #-}
 
 module T11640 where
 
diff --git a/testsuite/tests/typecheck/should_compile/tc247.hs b/testsuite/tests/typecheck/should_compile/tc247.hs
index 0f017a02db..abfc9ac9a4 100644
--- a/testsuite/tests/typecheck/should_compile/tc247.hs
+++ b/testsuite/tests/typecheck/should_compile/tc247.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE EmptyDataDecls, KindSignatures #-}
+{-# LANGUAGE GADTs, EmptyDataDecls, KindSignatures #-}
 
 module ShouldCompile where