Fix DeriveAnyClass (again)

This patch fixes Trac #13272.  The general approach was fine, but
we were simply not generating the correct implication constraint
(in particular generating fresh unification variables).  I added
a lot more commentary to Note [Gathering and simplifying
constraints for DeriveAnyClass]

I'm still not very happy with the overall architecture.  It feels
more complicate than it should.

3 files changed, 46 insertions, 0 deletions

diff --git a/testsuite/tests/deriving/should_compile/T13272.hs b/testsuite/tests/deriving/should_compile/T13272.hs
new file mode 100644
index 0000000000..8ce3368d1c
--- /dev/null
+++ b/testsuite/tests/deriving/should_compile/T13272.hs
@@ -0,0 +1,23 @@
+{-# LANGUAGE DefaultSignatures #-}
+{-# LANGUAGE DeriveAnyClass #-}
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeFamilies #-}
+module T13272 where
+
+import GHC.Generics
+
+class TypeName a where
+  typeName         :: forall proxy.
+                      proxy a -> String
+  default typeName :: forall proxy d f.
+                      (Generic a, Rep a ~ D1 d f, Datatype d)
+                   => proxy a -> String
+  typeName _ = gtypeName $ from (undefined :: a)
+
+gtypeName :: Datatype d => D1 d f p -> String
+gtypeName = datatypeName
+
+data T a = MkT a
+  deriving (Generic, TypeName)
diff --git a/testsuite/tests/deriving/should_compile/T13272a.hs b/testsuite/tests/deriving/should_compile/T13272a.hs
new file mode 100644
index 0000000000..3621879749
--- /dev/null
+++ b/testsuite/tests/deriving/should_compile/T13272a.hs
@@ -0,0 +1,21 @@
+{-# LANGUAGE DefaultSignatures #-}
+{-# LANGUAGE DeriveAnyClass #-}
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeFamilies #-}
+module T13272a where
+
+import GHC.Generics
+
+class TypeName a where
+  typeName         :: proxy a -> String
+  default typeName :: (Generic a, Rep a ~ gg, gg ~ D1 d f, Datatype d)
+                   => proxy a -> String
+  typeName _ = gtypeName $ from (undefined :: a)
+
+gtypeName :: Datatype d => D1 d f p -> String
+gtypeName = datatypeName
+
+data T a = MkT a
+  deriving (Generic, TypeName)
diff --git a/testsuite/tests/deriving/should_compile/all.T b/testsuite/tests/deriving/should_compile/all.T
index 288b3b7fdb..e16bd95034 100644
--- a/testsuite/tests/deriving/should_compile/all.T
+++ b/testsuite/tests/deriving/should_compile/all.T
@@ -82,3 +82,5 @@ test('T12594', normal, compile, [''])
 test('T12616', normal, compile, [''])
 test('T12688', normal, compile, [''])
 test('T12814', normal, compile, ['-Wredundant-constraints'])
+test('T13272', normal, compile, [''])
+test('T13272a', normal, compile, [''])