[commit: ghc] master: Comments only (403cfc9)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/403cfc9187b9df560768bb809f4d280fb999639c/ghc

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

commit 403cfc9187b9df560768bb809f4d280fb999639c
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Mon Jun 1 08:57:23 2015 +0100

    Comments only


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403cfc9187b9df560768bb809f4d280fb999639c
 compiler/typecheck/TcValidity.hs | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/compiler/typecheck/TcValidity.hs b/compiler/typecheck/TcValidity.hs
index 16059e6..370cad2 100644
--- a/compiler/typecheck/TcValidity.hs
+++ b/compiler/typecheck/TcValidity.hs
@@ -132,12 +132,19 @@ We call checkAmbiguity
    (b) in checkValidType
 
 Conncerning (b), you might wonder about nested foralls.  What about
-    f :: (forall a. Eq a => Int) -> Int
+    f :: forall b. (forall a. Eq a => b) -> b
 The nested forall is ambiguous.  Originally we called checkAmbiguity
-in the forall case of check_type, but that had a bad consequence:
-we got two error messages about (Eq b) in a nested forall like this:
-    g :: forall a. Eq a => forall b. Eq b => a -> a
+in the forall case of check_type, but that had two bad consequences:
+  * We got two error messages about (Eq b) in a nested forall like this:
+       g :: forall a. Eq a => forall b. Eq b => a -> a
+  * If we try to check for ambiguity of an nested forall like
+    (forall a. Eq a => b), the implication constraint doesn't bind
+    all the skolems, which results in "No skolem info" in error
+    messages (see Trac #10432).
+
 To avoid this, we call checkAmbiguity once, at the top, in checkValidType.
+(I'm still a bit worried about unbound skolems when the type mentions
+in-scope type variables.)
 
 In fact, because of the co/contra-variance implemented in tcSubType,
 this *does* catch function f above. too.