Refactor (~) to reduce the suerpclass stack

The constraint (~) used to be (effectively):
  class a ~~ b => (a :: k) ~ (b :: k)

but, with this patch, it is now defined uniformly with
(~~) and Coercible like this:
  class a ~# b => (a :: k) ~ (b :: k)

Result:
  * One less superclass selection when goinng from (~) to (~#)
    Better for compile time and better for debugging with -ddump-simpl

  * The code for (~), (~~), and Coercible looks uniform, and appears
    together, e.g. in TysWiredIn and ClsInst.matchGlobalInst.
    Previously the code for (~) was different, and unique.

Not only is this simpler, but it also makes the compiler a bit faster;
  T12227: 9% less allocation
  T12545: 7% less allocation

This patch fixes Trac #15421

1 files changed, 4 insertions, 9 deletions

diff --git a/testsuite/tests/ghci/scripts/T10059.stdout b/testsuite/tests/ghci/scripts/T10059.stdout
index 26e1e7e28f..92fbb45ef7 100644
--- a/testsuite/tests/ghci/scripts/T10059.stdout
+++ b/testsuite/tests/ghci/scripts/T10059.stdout
@@ -1,9 +1,4 @@
-class (a ~ b) => (~) (a :: k) (b :: k)
-  	-- Defined in ‘Data.Type.Equality’
-instance [incoherent] forall k (a :: k) (b :: k). (a ~ b) => a ~ b
-  -- Defined in ‘Data.Type.Equality’
-(~) :: k -> k -> Constraint
-class (a ~~ b) => (~) (a :: k) (b :: k)
-  	-- Defined in ‘Data.Type.Equality’
-instance [incoherent] forall k (a :: k) (b :: k). (a ~~ b) => a ~ b
-  -- Defined in ‘Data.Type.Equality’
+class (a ~ b) => (~) (a :: k0) (b :: k0) 	-- Defined in ‘GHC.Types’
+(~) :: k0 -> k0 -> Constraint
+class (a GHC.Prim.~# b) => (~) (a :: k0) (b :: k0)
+  	-- Defined in ‘GHC.Types’