Fix two constraint solving problems

This patch fixes two problems in the constraint solver.

* An actual bug #18555: we were floating out a constraint to eagerly,
  and that was ultimately fatal.  It's explained in
  Note [Do not float blocked constraints] in GHC.Core.Constraint.

  This is all very delicate, but it's all going to become irrelevant
  when we stop floating constraints (#17656).

* A major performance infelicity in the flattener.  When flattening
  (ty |> co) we *never* generated Refl, even when there was nothing
  at all to do.  Result: we would gratuitously rewrite the constraint
  to exactly the same thing, wasting work.  Described in #18413, and
  came up again in #18855.

  Solution: exploit the special case by calling the new function
  castCoercionKind1.  See Note [castCoercionKind1] in
  GHC.Core.Coercion

2 files changed, 19 insertions, 0 deletions

diff --git a/testsuite/tests/polykinds/T18855.hs b/testsuite/tests/polykinds/T18855.hs
new file mode 100644
index 0000000000..131cd9c8e7
--- /dev/null
+++ b/testsuite/tests/polykinds/T18855.hs
@@ -0,0 +1,18 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE StandaloneKindSignatures #-}
+{-# LANGUAGE TypeFamilies #-}
+module Bug where
+
+import Data.Kind
+
+type family Apply (f :: a -> b) (x :: a) :: b
+
+type F :: forall a.
+          forall (p :: forall bOne. Either a bOne -> Type)
+       -> forall bTwo.
+          forall (e :: Either a bTwo)
+       -> Apply p e
+
+type family F
diff --git a/testsuite/tests/polykinds/all.T b/testsuite/tests/polykinds/all.T
index b167b930dc..009172537b 100644
--- a/testsuite/tests/polykinds/all.T
+++ b/testsuite/tests/polykinds/all.T
@@ -225,3 +225,4 @@ test('T18451', normal, compile_fail, [''])
 test('T18451a', normal, compile_fail, [''])
 test('T18451b', normal, compile_fail, [''])
 test('T18522-ppr', normal, ghci_script, ['T18522-ppr.script'])
+test('T18855', normal, compile, [''])