Add a warning for GADT match + NoMonoLocalBinds (#20485)

Previously, it was an error to pattern match on a GADT
without GADTs or TypeFamilies.
This is now allowed. Instead, we check the flag MonoLocalBinds;
if it is not enabled, we issue a warning, controlled by -Wgadt-mono-local-binds.

Also fixes #20485: pattern synonyms are now checked too.

6 files changed, 43 insertions, 1 deletions

diff --git a/testsuite/tests/gadt/T20485.hs b/testsuite/tests/gadt/T20485.hs
new file mode 100644
index 0000000000..5fb115df6e
--- /dev/null
+++ b/testsuite/tests/gadt/T20485.hs
@@ -0,0 +1,12 @@
+{-# LANGUAGE PatternSynonyms #-}
+module T20485 where
+
+import Data.Type.Equality
+
+f :: a :~: b -> a -> b
+f Refl x = x
+
+pattern ReflPat = Refl
+
+g :: a :~: b -> a -> b
+g ReflPat x = x
diff --git a/testsuite/tests/gadt/T20485.stderr b/testsuite/tests/gadt/T20485.stderr
new file mode 100644
index 0000000000..e140b58ad3
--- /dev/null
+++ b/testsuite/tests/gadt/T20485.stderr
@@ -0,0 +1,15 @@
+
+T20485.hs:7:3: warning: [-Wgadt-mono-local-binds (in -Wdefault)]
+    Pattern matching on GADTs without MonoLocalBinds is fragile.
+    Suggested fix:
+      Enable any of the following extensions: GADTs, TypeFamilies
+
+T20485.hs:9:19: warning: [-Wgadt-mono-local-binds (in -Wdefault)]
+    Pattern matching on GADTs without MonoLocalBinds is fragile.
+    Suggested fix:
+      Enable any of the following extensions: GADTs, TypeFamilies
+
+T20485.hs:12:3: warning: [-Wgadt-mono-local-binds (in -Wdefault)]
+    Pattern matching on GADTs without MonoLocalBinds is fragile.
+    Suggested fix:
+      Enable any of the following extensions: GADTs, TypeFamilies
diff --git a/testsuite/tests/gadt/T20485a.hs b/testsuite/tests/gadt/T20485a.hs
new file mode 100644
index 0000000000..fa8c1ee969
--- /dev/null
+++ b/testsuite/tests/gadt/T20485a.hs
@@ -0,0 +1,12 @@
+{-# LANGUAGE PatternSynonyms, MonoLocalBinds #-}
+module T20485a where
+
+import Data.Type.Equality
+
+f :: a :~: b -> a -> b
+f Refl x = x
+
+pattern ReflPat = Refl
+
+g :: a :~: b -> a -> b
+g ReflPat x = x
diff --git a/testsuite/tests/gadt/all.T b/testsuite/tests/gadt/all.T
index 9179a40288..3152f3561b 100644
--- a/testsuite/tests/gadt/all.T
+++ b/testsuite/tests/gadt/all.T
@@ -122,3 +122,5 @@ test('T16427', normal, compile_fail, [''])
 test('T18191', normal, compile_fail, [''])
 test('T20278', normal, compile, [''])
 test('SynDataRec', normal, compile, [''])
+test('T20485', normal, compile, [''])
+test('T20485a', normal, compile, [''])
diff --git a/testsuite/tests/patsyn/should_compile/T10997.hs b/testsuite/tests/patsyn/should_compile/T10997.hs
index 69a7940a5f..49278a2a33 100644
--- a/testsuite/tests/patsyn/should_compile/T10997.hs
+++ b/testsuite/tests/patsyn/should_compile/T10997.hs
@@ -1,3 +1,4 @@
+{-# OPTIONS_GHC -Wno-gadt-mono-local-binds #-}
 module T10997 where
 
 import T10997a
diff --git a/testsuite/tests/typecheck/should_run/Typeable1.hs b/testsuite/tests/typecheck/should_run/Typeable1.hs
index 02a7ebb98b..9a38c85927 100644
--- a/testsuite/tests/typecheck/should_run/Typeable1.hs
+++ b/testsuite/tests/typecheck/should_run/Typeable1.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE PolyKinds, GADTs #-}
 
 import Type.Reflection
 import Data.Kind