Suggest how to fix illegally nested foralls in GADT constructor type signatures

Summary:
Although the code from #12087 isn't accepted by GHC, we can at least
do a better job of letting users know what the problem is, and how to fix it.

Test Plan: make test TEST=T12087

Reviewers: goldfire, austin, bgamari

Reviewed By: goldfire

Subscribers: rwbarton, thomie

GHC Trac Issues: #12087

Differential Revision: https://phabricator.haskell.org/D3851

3 files changed, 55 insertions, 1 deletions

diff --git a/testsuite/tests/gadt/T12087.hs b/testsuite/tests/gadt/T12087.hs
new file mode 100644
index 0000000000..e56240c2a4
--- /dev/null
+++ b/testsuite/tests/gadt/T12087.hs
@@ -0,0 +1,18 @@
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE RankNTypes #-}
+module T12087 where
+
+data F1 a where
+  MkF1 :: Ord a => Eq a => a -> F1 a
+
+data F2 a where
+  MkF2 :: Ord a => a -> Eq a => F2 a
+
+data F3 a where
+  MkF3 :: forall a. Eq a => a -> forall b. Eq b => b -> F3 a
+
+data F4 a where
+  MkF4 :: forall a b. Eq a => a -> Eq b => b -> F4 a
+
+data F5 a where
+  MkF5 :: Int -> Int -> forall a. a -> Int -> Int -> forall b. b -> F5 a
diff --git a/testsuite/tests/gadt/T12087.stderr b/testsuite/tests/gadt/T12087.stderr
new file mode 100644
index 0000000000..03f2465c7a
--- /dev/null
+++ b/testsuite/tests/gadt/T12087.stderr
@@ -0,0 +1,35 @@
+
+T12087.hs:6:3: error:
+    • GADT constructor type signature cannot contain nested ‘forall’s or contexts
+      Suggestion: instead use this type signature:
+        MkF1 :: forall a. (Ord a, Eq a) => a -> F1 a
+    • In the definition of data constructor ‘MkF1’
+      In the data type declaration for ‘F1’
+
+T12087.hs:9:3: error:
+    • GADT constructor type signature cannot contain nested ‘forall’s or contexts
+      Suggestion: instead use this type signature:
+        MkF2 :: forall a. (Ord a, Eq a) => F2 a
+    • In the definition of data constructor ‘MkF2’
+      In the data type declaration for ‘F2’
+
+T12087.hs:12:3: error:
+    • GADT constructor type signature cannot contain nested ‘forall’s or contexts
+      Suggestion: instead use this type signature:
+        MkF3 :: forall a b. (Eq a, Eq b) => b -> F3 a
+    • In the definition of data constructor ‘MkF3’
+      In the data type declaration for ‘F3’
+
+T12087.hs:15:3: error:
+    • GADT constructor type signature cannot contain nested ‘forall’s or contexts
+      Suggestion: instead use this type signature:
+        MkF4 :: forall a b. (Eq a, Eq b) => b -> F4 a
+    • In the definition of data constructor ‘MkF4’
+      In the data type declaration for ‘F4’
+
+T12087.hs:18:3: error:
+    • GADT constructor type signature cannot contain nested ‘forall’s or contexts
+      Suggestion: instead use this type signature:
+        MkF5 :: forall a b. a -> Int -> Int -> b -> F5 a
+    • In the definition of data constructor ‘MkF5’
+      In the data type declaration for ‘F5’
diff --git a/testsuite/tests/gadt/all.T b/testsuite/tests/gadt/all.T
index 877943b0c5..3c825f0848 100644
--- a/testsuite/tests/gadt/all.T
+++ b/testsuite/tests/gadt/all.T
@@ -56,7 +56,7 @@ test('records-fail1', normal, compile_fail, [''])
 test('doaitse', normal, compile, [''])
 test('josef', normal, compile, [''])
 
-# Interaction of fundeps with GADTs doesn't work well 
+# Interaction of fundeps with GADTs doesn't work well
 test('gadt-fd', expect_broken(345), compile, [''])
 
 test('karl1', normal, compile, [''])
@@ -112,4 +112,5 @@ test('T7974', normal, compile, [''])
 test('T7558', normal, compile_fail, [''])
 test('T9096', normal, compile, [''])
 test('T9380', normal, compile_and_run, [''])
+test('T12087', normal, compile_fail, [''])
 test('T12468', normal, compile_fail, [''])