Allow (~) in the head of a quantified constraints

Since the introduction of quantified constraints, GHC has rejected
a quantified constraint with (~) in the head, thus
  f :: (forall a. blah => a ~ ty) => stuff

I am frankly dubious that this is ever useful.  But /is/ necessary for
Coercible (representation equality version of (~)) and it does no harm
to allow it for (~) as well.  Plus, our users are asking for it
(Trac #15359, #15625).

It was really only excluded by accident, so
this patch lifts the restriction. See TcCanonical
Note [Equality superclasses in quantified constraints]

There are a number of wrinkles:

* If the context of the quantified constraint is empty, we
  can get trouble when we get down to unboxed equality (a ~# b)
  or (a ~R# b), as Trac #15625 showed. This is even more of
  a corner case, but it produced an outright crash, so I elaborated
  the superclass machinery in TcCanonical.makeStrictSuperClasses
  to add a void argument in this case.  See
  Note [Equality superclasses in quantified constraints]

* The restriction on (~) was in TcValidity.checkValidInstHead.
  In lifting the restriction I discovered an old special case for
  (~), namely
      | clas_nm `elem` [ heqTyConName, eqTyConName]
      , nameModule clas_nm /= this_mod
  This was (solely) to support the strange instance
      instance a ~~ b => a ~ b
  in Data.Type.Equality. But happily that is no longer
  with us, since
     commit f265008fb6f70830e7e92ce563f6d83833cef071
     Refactor (~) to reduce the suerpclass stack
  So I removed the special case.

* I found that the Core invariants on when we could have
       co = <expr>
  were entirely not written down. (Getting this wrong ws
  the proximate source of the crash in Trac #15625.  So

  - Documented them better in CoreSyn
      Note [CoreSyn type and coercion invariant],
  - Modified CoreOpt and CoreLint to match
  - Modified CoreUtils.bindNonRec to match
  - Made MkCore.mkCoreLet use bindNonRec, rather
    than duplicate its logic
  - Made Simplify.rebuildCase case-to-let respect
      Note [CoreSyn type and coercion invariant],

5 files changed, 66 insertions, 0 deletions

diff --git a/testsuite/tests/quantified-constraints/T15359.hs b/testsuite/tests/quantified-constraints/T15359.hs
new file mode 100644
index 0000000000..7ef1cc5572
--- /dev/null
+++ b/testsuite/tests/quantified-constraints/T15359.hs
@@ -0,0 +1,12 @@
+{-# LANGUAGE MultiParamTypeClasses, GADTs, RankNTypes,
+             ConstraintKinds, QuantifiedConstraints #-}
+
+module T15359 where
+
+class C a b
+
+data Dict c where
+  Dict :: c => Dict c
+
+foo :: (forall a b. C a b => a ~ b) => Dict (C a b) -> a -> b
+foo Dict x = x
diff --git a/testsuite/tests/quantified-constraints/T15359a.hs b/testsuite/tests/quantified-constraints/T15359a.hs
new file mode 100644
index 0000000000..6ec5f861a8
--- /dev/null
+++ b/testsuite/tests/quantified-constraints/T15359a.hs
@@ -0,0 +1,14 @@
+{-# LANGUAGE MultiParamTypeClasses, GADTs, RankNTypes,
+             ConstraintKinds, QuantifiedConstraints,
+             UndecidableInstances #-}
+
+module T15359a where
+
+class C a b
+class a ~ b => D a b
+
+data Dict c where
+  Dict :: c => Dict c
+
+foo :: (forall a b. C a b => D a b) => Dict (C a b) -> a -> b
+foo Dict x = x
diff --git a/testsuite/tests/quantified-constraints/T15625.hs b/testsuite/tests/quantified-constraints/T15625.hs
new file mode 100644
index 0000000000..8fe092da98
--- /dev/null
+++ b/testsuite/tests/quantified-constraints/T15625.hs
@@ -0,0 +1,16 @@
+{-# Language GADTs, MultiParamTypeClasses, QuantifiedConstraints #-}
+
+module T15625 where
+
+import Data.Coerce
+
+class a ~ b => Equal a b
+
+test1 :: (forall b. a ~ b) => a
+test1 = False
+
+test2 :: (forall b. Equal a b) => a
+test2 = False
+
+test3 :: (forall b. Coercible a b) => a
+test3 = coerce False
diff --git a/testsuite/tests/quantified-constraints/T15625a.hs b/testsuite/tests/quantified-constraints/T15625a.hs
new file mode 100644
index 0000000000..ca049cb19d
--- /dev/null
+++ b/testsuite/tests/quantified-constraints/T15625a.hs
@@ -0,0 +1,20 @@
+{-# Language RankNTypes, ConstraintKinds, QuantifiedConstraints,
+             PolyKinds, GADTs, MultiParamTypeClasses,
+             DataKinds, FlexibleInstances #-}
+
+module T15625a where
+
+import Data.Kind
+
+type Cat ob = ob -> ob -> Type
+
+data KLEISLI (m :: Type -> Type) :: Cat (KL_kind m) where
+  MkKLEISLI :: (a -> m b) -> KLEISLI(m) (KL a) (KL b)
+
+data KL_kind (m :: Type -> Type) = KL Type
+
+class    (a ~ KL xx) => AsKL a xx
+instance (a ~ KL xx) => AsKL a xx
+
+ekki__ :: Monad m => (forall xx. AsKL a xx) => KLEISLI m a a
+ekki__ = MkKLEISLI undefined
diff --git a/testsuite/tests/quantified-constraints/all.T b/testsuite/tests/quantified-constraints/all.T
index 833a667ea9..1e2eca8358 100644
--- a/testsuite/tests/quantified-constraints/all.T
+++ b/testsuite/tests/quantified-constraints/all.T
@@ -16,3 +16,7 @@ test('T15290a', normal, compile_fail, [''])
 test('T15290b', normal, compile_fail, [''])
 test('T15316', normal, compile_fail, [''])
 test('T15334', normal, compile_fail, [''])
+test('T15359', normal, compile, [''])
+test('T15359a', normal, compile, [''])
+test('T15625', normal, compile, [''])
+test('T15625a', normal, compile, [''])