[commit: ghc] ghc-7.10: Revert "Look inside synonyms for foralls when unifying" (8fb101e)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : ghc-7.10
Link       : http://ghc.haskell.org/trac/ghc/changeset/8fb101e49b86c0f8bb8931620c9c3cd3e6c57228/ghc

>---------------------------------------------------------------

commit 8fb101e49b86c0f8bb8931620c9c3cd3e6c57228
Author: Austin Seipp <austin at well-typed.com>
Date:   Thu Jun 18 10:34:28 2015 -0500

    Revert "Look inside synonyms for foralls when unifying"
    
    As discussed in #10194, this patch - while fixing a bug - also causes a
    minor regression when compiling certain in-the-wild programs, meaning
    some extant programs now failed with 7.10.2 RC1. Womp womp.
    
    This reverts commit 681d82c0d44f06f0b958b75778c30b0910df982b.


>---------------------------------------------------------------

8fb101e49b86c0f8bb8931620c9c3cd3e6c57228
 compiler/typecheck/TcType.hs                        | 10 +++-------
 compiler/typecheck/TcUnify.hs                       |  5 +----
 testsuite/tests/typecheck/should_fail/T10194.hs     |  7 -------
 testsuite/tests/typecheck/should_fail/T10194.stderr |  7 -------
 testsuite/tests/typecheck/should_fail/all.T         |  1 -
 5 files changed, 4 insertions(+), 26 deletions(-)

diff --git a/compiler/typecheck/TcType.hs b/compiler/typecheck/TcType.hs
index 41db197..a5a5075 100644
--- a/compiler/typecheck/TcType.hs
+++ b/compiler/typecheck/TcType.hs
@@ -853,7 +853,7 @@ mkTcEqPredRole Nominal          = mkTcEqPred
 mkTcEqPredRole Representational = mkTcReprEqPred
 mkTcEqPredRole Phantom          = panic "mkTcEqPredRole Phantom"
 
--- @isTauTy@ tests for nested for-alls.
+-- @isTauTy@ tests for nested for-alls.  It should not be called on a boxy type.
 
 isTauTy :: Type -> Bool
 isTauTy ty | Just ty' <- tcView ty = isTauTy ty'
@@ -1228,7 +1228,7 @@ occurCheckExpand dflags tv ty
     -- True => fine
     fast_check (LitTy {})        = True
     fast_check (TyVarTy tv')     = tv /= tv'
-    fast_check (TyConApp tc tys) = all fast_check tys && (isTauTyCon tc || impredicative)
+    fast_check (TyConApp _ tys)  = all fast_check tys
     fast_check (FunTy arg res)   = fast_check arg && fast_check res
     fast_check (AppTy fun arg)   = fast_check fun && fast_check arg
     fast_check (ForAllTy tv' ty) = impredicative
@@ -1262,11 +1262,7 @@ occurCheckExpand dflags tv ty
     -- it and try again.
     go ty@(TyConApp tc tys)
       = case do { tys <- mapM go tys; return (mkTyConApp tc tys) } of
-          OC_OK ty
-              | impredicative || isTauTyCon tc
-              -> return ty  -- First try to eliminate the tyvar from the args
-              | otherwise
-              -> OC_Forall  -- A type synonym with a forall on the RHS
+          OC_OK ty -> return ty  -- First try to eliminate the tyvar from the args
           bad | Just ty' <- tcView ty -> go ty'
               | otherwise             -> bad
                       -- Failing that, try to expand a synonym
diff --git a/compiler/typecheck/TcUnify.hs b/compiler/typecheck/TcUnify.hs
index ef21d87..b4a6ada 100644
--- a/compiler/typecheck/TcUnify.hs
+++ b/compiler/typecheck/TcUnify.hs
@@ -1012,13 +1012,10 @@ checkTauTvUpdate dflags tv ty
     defer_me :: TcType -> Bool
     -- Checks for (a) occurrence of tv
     --            (b) type family applications
-    --            (c) foralls
     -- See Note [Conservative unification check]
     defer_me (LitTy {})        = False
     defer_me (TyVarTy tv')     = tv == tv'
-    defer_me (TyConApp tc tys) = isTypeFamilyTyCon tc
-                                 || any defer_me tys
-                                 || not (impredicative || isTauTyCon tc)
+    defer_me (TyConApp tc tys) = isTypeFamilyTyCon tc || any defer_me tys
     defer_me (FunTy arg res)   = defer_me arg || defer_me res
     defer_me (AppTy fun arg)   = defer_me fun || defer_me arg
     defer_me (ForAllTy _ ty)   = not impredicative || defer_me ty
diff --git a/testsuite/tests/typecheck/should_fail/T10194.hs b/testsuite/tests/typecheck/should_fail/T10194.hs
deleted file mode 100644
index 2174a59..0000000
--- a/testsuite/tests/typecheck/should_fail/T10194.hs
+++ /dev/null
@@ -1,7 +0,0 @@
-{-# LANGUAGE RankNTypes #-}
-module T10194 where
-
-type X = forall a . a
-
-comp :: (X -> c) -> (a -> X) -> (a -> c)
-comp = (.)
diff --git a/testsuite/tests/typecheck/should_fail/T10194.stderr b/testsuite/tests/typecheck/should_fail/T10194.stderr
deleted file mode 100644
index 53ee74b..0000000
--- a/testsuite/tests/typecheck/should_fail/T10194.stderr
+++ /dev/null
@@ -1,7 +0,0 @@
-
-T10194.hs:7:8:
-    Cannot instantiate unification variable ‘b0’
-    with a type involving foralls: X
-      Perhaps you want ImpredicativeTypes
-    In the expression: (.)
-    In an equation for ‘comp’: comp = (.)
diff --git a/testsuite/tests/typecheck/should_fail/all.T b/testsuite/tests/typecheck/should_fail/all.T
index 31b6a5f..95911d1 100644
--- a/testsuite/tests/typecheck/should_fail/all.T
+++ b/testsuite/tests/typecheck/should_fail/all.T
@@ -354,7 +354,6 @@ test('T8044', normal, compile_fail, [''])
 test('T4921', normal, compile_fail, [''])
 test('T9605', normal, compile_fail, [''])
 test('T9999', normal, compile_fail, [''])
-test('T10194', normal, compile_fail, [''])
 
 test('T10285',
      extra_clean(['T10285a.hi', 'T10285a.o']),