Disallow non-nullary constraint synonyms on class.

Test Plan: validate

Reviewers: simonpj, bgamari, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D3230

4 files changed, 40 insertions, 4 deletions

diff --git a/testsuite/tests/backpack/should_compile/bkp39.bkp b/testsuite/tests/backpack/should_compile/bkp39.bkp
index bf98b10c96..a6216edc24 100644
--- a/testsuite/tests/backpack/should_compile/bkp39.bkp
+++ b/testsuite/tests/backpack/should_compile/bkp39.bkp
@@ -3,7 +3,6 @@ unit p where
     signature A where
         import Prelude hiding ((==))
         class K a
-        class K2 a
         infix 4 ==
         (==) :: K a => a -> a -> Bool
     module M where
@@ -11,8 +10,7 @@ unit p where
         import A
         f a b c = a == b && b == c
 unit q where
-    module A(K, K2, (==)) where
-        type K a = Eq a
-        type K2 = Eq
+    module A(K, (==)) where
+        type K = Eq
 unit r where
     dependency p[A=q:A]
diff --git a/testsuite/tests/backpack/should_fail/all.T b/testsuite/tests/backpack/should_fail/all.T
index 77b7aa23d8..82b4e6803b 100644
--- a/testsuite/tests/backpack/should_fail/all.T
+++ b/testsuite/tests/backpack/should_fail/all.T
@@ -41,3 +41,4 @@ test('bkpfail42', normal, backpack_compile_fail, [''])
 test('bkpfail43', normal, backpack_compile_fail, [''])
 test('bkpfail44', normal, backpack_compile_fail, [''])
 test('bkpfail45', normal, backpack_compile_fail, [''])
+test('bkpfail46', normal, backpack_compile_fail, [''])
diff --git a/testsuite/tests/backpack/should_fail/bkpfail46.bkp b/testsuite/tests/backpack/should_fail/bkpfail46.bkp
new file mode 100644
index 0000000000..0fc8b676a8
--- /dev/null
+++ b/testsuite/tests/backpack/should_fail/bkpfail46.bkp
@@ -0,0 +1,17 @@
+{-# LANGUAGE ConstraintKinds #-}
+unit p where
+    signature A where
+        import Prelude hiding ((==))
+        class K a
+        infix 4 ==
+        (==) :: K a => a -> a -> Bool
+    module M where
+        import Prelude hiding ((==))
+        import A
+        f a b c = a == b && b == c
+unit q where
+    module A(K, (==)) where
+        -- This won't match because it's not nullary
+        type K a = Eq a
+unit r where
+    dependency p[A=q:A]
diff --git a/testsuite/tests/backpack/should_fail/bkpfail46.stderr b/testsuite/tests/backpack/should_fail/bkpfail46.stderr
new file mode 100644
index 0000000000..9aeaccbb7e
--- /dev/null
+++ b/testsuite/tests/backpack/should_fail/bkpfail46.stderr
@@ -0,0 +1,20 @@
+[1 of 3] Processing p
+  [1 of 2] Compiling A[sig]           ( p/A.hsig, nothing )
+  [2 of 2] Compiling M                ( p/M.hs, nothing )
+[2 of 3] Processing q
+  Instantiating q
+  [1 of 1] Compiling A                ( q/A.hs, bkpfail46.out/q/A.o )
+[3 of 3] Processing r
+  Instantiating r
+  [1 of 1] Including p[A=q:A]
+    Instantiating p[A=q:A]
+    [1 of 2] Compiling A[sig]           ( p/A.hsig, bkpfail46.out/p/p-HVmFlcYSefiK5n1aDP1v7x/A.o )
+
+bkpfail46.bkp:15:9: error:
+    • Type constructor ‘K’ has conflicting definitions in the module
+      and its hsig file
+      Main module: type K a = GHC.Classes.Eq a :: Constraint
+      Hsig file:  class K a
+      Illegal parameterized type synonym in implementation of abstract data.
+      (Try eta reducing your type synonym so that it is nullary.)
+    • while checking that q:A implements signature A in p[A=q:A]