[commit: ghc] master: A bit more tc-tracking in TcUnify.uUnfilledVar (7a2b5d0)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/7a2b5d0c97da71f5e5a7d82b28e8a734b0ff95df/ghc

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

commit 7a2b5d0c97da71f5e5a7d82b28e8a734b0ff95df
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Tue Jun 26 12:07:08 2018 +0100

    A bit more tc-tracking in TcUnify.uUnfilledVar


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

7a2b5d0c97da71f5e5a7d82b28e8a734b0ff95df
 compiler/typecheck/TcUnify.hs | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/compiler/typecheck/TcUnify.hs b/compiler/typecheck/TcUnify.hs
index fa845bb..9b32c3c 100644
--- a/compiler/typecheck/TcUnify.hs
+++ b/compiler/typecheck/TcUnify.hs
@@ -1571,19 +1571,25 @@ uUnfilledVar2 origin t_or_k swapped tv1 ty2
       | canSolveByUnification cur_lvl tv1 ty2
       , Just ty2' <- metaTyVarUpdateOK dflags tv1 ty2
       = do { co_k <- uType KindLevel kind_origin (typeKind ty2') (tyVarKind tv1)
+           ; traceTc "uUnfilledVar2 ok" $
+             vcat [ ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1)
+                  , ppr ty2 <+> dcolon <+> ppr (typeKind  ty2)
+                  , ppr (isTcReflCo co_k), ppr co_k ]
+
            ; if isTcReflCo co_k  -- only proceed if the kinds matched.
 
              then do { writeMetaTyVar tv1 ty2'
                      ; return (mkTcNomReflCo ty2') }
-             else defer } -- this cannot be solved now.
-                          -- See Note [Equalities with incompatible kinds]
-                          -- in TcCanonical
+
+             else defer } -- This cannot be solved now.  See TcCanonical
+                          -- Note [Equalities with incompatible kinds]
 
       | otherwise
-      = defer
+      = do { traceTc "uUnfilledVar2 not ok" (ppr tv1 $$ ppr ty2)
                -- Occurs check or an untouchable: just defer
                -- NB: occurs check isn't necessarily fatal:
                --     eg tv1 occured in type family parameter
+            ; defer }
 
     ty1 = mkTyVarTy tv1
     kind_origin = KindEqOrigin ty1 (Just ty2) origin (Just t_or_k)