Type checking for type synonym families

[ghc-hetmet.git] / compiler / typecheck / TcBinds.lhs

diff --git a/compiler/typecheck/TcBinds.lhs b/compiler/typecheck/TcBinds.lhs
index 93a9010..35c5c2c 100644 (file)
--- a/compiler/typecheck/TcBinds.lhs
+++ b/compiler/typecheck/TcBinds.lhs
@@ -30,6 +30,7 @@ import TcPat
 import TcMType
 import TcType
 import {- Kind parts of -} Type
+import Coercion
 import VarEnv
 import TysPrim
 import Id
@@ -241,7 +242,7 @@ tc_haskell98 top_lvl sig_fn prag_fn rec_flag binds thing_inside
 bindLocalInsts :: TopLevelFlag -> TcM ([LHsBinds TcId], [TcId], a) -> TcM ([LHsBinds TcId], a)
 bindLocalInsts top_lvl thing_inside
   | isTopLevel top_lvl = do { (binds, ids, thing) <- thing_inside; return (binds, thing) }
-       -- For the top level don't bother will all this bindInstsOfLocalFuns stuff. 
+       -- For the top level don't bother with all this bindInstsOfLocalFuns stuff. 
        -- All the top level things are rec'd together anyway, so it's fine to
        -- leave them to the tcSimplifyTop, and quite a bit faster too
 
@@ -769,7 +770,17 @@ unifyCtxts (sig1 : sigs)   -- Argument is always non-empty
     unify_ctxt sig@(TcSigInfo { sig_theta = theta })
        = setSrcSpan (instLocSpan (sig_loc sig))        $
          addErrCtxt (sigContextsCtxt sig1 sig)         $
-         unifyTheta theta1 theta
+         do { cois <- unifyTheta theta1 theta
+            ; -- Check whether all coercions are identity coercions
+              -- That can happen if we have, say
+              --         f :: C [a]   => ...
+              --         g :: C (F a) => ...
+              -- where F is a type function and (F a ~ [a])
+              -- Then unification might succeed with a coercion.  But it's much
+              -- much simpler to require that such signatures have identical contexts
+              checkTc (all isIdentityCoercion cois)
+                      (ptext SLIT("Mutually dependent functions have syntactically distinct contexts"))
+            }
 
 checkSigsTyVars :: [TcTyVar] -> [TcSigInfo] -> TcM [TcTyVar]
 checkSigsTyVars qtvs sigs