[commit: ghc] wip/custom-type-errors: Check that inferred and user-specified types od not mention `TypeError` directly (064875e)

[git at git.haskell.org]

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

On branch  : wip/custom-type-errors
Link       : http://ghc.haskell.org/trac/ghc/changeset/064875e5ccddb3c3190c4f03f93763bafad2ef3a/ghc

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

commit 064875e5ccddb3c3190c4f03f93763bafad2ef3a
Author: Iavor S. Diatchki <iavor.diatchki at gmail.com>
Date:   Sat Oct 31 13:18:44 2015 -0700

    Check that inferred and user-specified types od not mention `TypeError` directly


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

064875e5ccddb3c3190c4f03f93763bafad2ef3a
 compiler/typecheck/TcValidity.hs | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/compiler/typecheck/TcValidity.hs b/compiler/typecheck/TcValidity.hs
index 62449be..3b5d206 100644
--- a/compiler/typecheck/TcValidity.hs
+++ b/compiler/typecheck/TcValidity.hs
@@ -232,6 +232,16 @@ checkAmbiguity ctxt ty
        mk_msg ty = pprSigCtxt ctxt (ptext (sLit "the ambiguity check for")) (ppr ty)
        ambig_msg = ptext (sLit "To defer the ambiguity check to use sites, enable AllowAmbiguousTypes")
 
+
+checkUserTypeError :: Type -> TcM ()
+checkUserTypeError = check
+  where
+  check ty
+    | Just (_,msg) <- isUserErrorTy ty = failWithTc (pprUserTypeErrorTy msg)
+    | Just (_,ts)  <- splitTyConApp_maybe ty  = mapM_ check ts
+    | Just (t1,t2) <- splitAppTy_maybe ty     = check t1 >> check t2
+    | otherwise                               = return ()
+
 {-
 ************************************************************************
 *                                                                      *
@@ -312,6 +322,8 @@ checkValidType ctxt ty
         -- the kind of an ill-formed type such as (a~Int)
        ; check_kind ctxt ty
 
+       ; checkUserTypeError ty
+
        -- Check for ambiguous types.  See Note [When to call checkAmbiguity]
        -- NB: this will happen even for monotypes, but that should be cheap;
        --     and there may be nested foralls for the subtype test to examine