diff options
Diffstat (limited to 'compiler/typecheck/TcUnify.hs')
| -rw-r--r-- | compiler/typecheck/TcUnify.hs | 32 |
1 files changed, 30 insertions, 2 deletions
diff --git a/compiler/typecheck/TcUnify.hs b/compiler/typecheck/TcUnify.hs index dcc185cb27..2ed861c327 100644 --- a/compiler/typecheck/TcUnify.hs +++ b/compiler/typecheck/TcUnify.hs @@ -758,8 +758,9 @@ tc_sub_type_ds eq_orig inst_orig ctxt ty_actual ty_expected | not (isPredTy act_arg) , not (isPredTy exp_arg) = -- See Note [Co/contra-variance of subsumption checking] - do { res_wrap <- tc_sub_type_ds eq_orig inst_orig ctxt act_res exp_res - ; arg_wrap <- tc_sub_tc_type eq_orig given_orig ctxt exp_arg act_arg + do { res_wrap <- tc_sub_type_ds eq_orig inst_orig ctxt act_res exp_res + ; arg_wrap <- tc_sub_tc_type eq_orig given_orig GenSigCtxt exp_arg act_arg + -- GenSigCtxt: See Note [Setting the argument context] ; return (mkWpFun arg_wrap res_wrap exp_arg exp_res doc) } -- arg_wrap :: exp_arg ~> act_arg -- res_wrap :: act-res ~> exp_res @@ -808,6 +809,33 @@ tc_sub_type_ds eq_orig inst_orig ctxt ty_actual ty_expected -- use versions without synonyms expanded unify = mkWpCastN <$> uType TypeLevel eq_orig ty_actual ty_expected +{- Note [Settting the argument context] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider we are doing the ambiguity check for the (bogus) + f :: (forall a b. C b => a -> a) -> Int + +We'll call + tcSubType ((forall a b. C b => a->a) -> Int ) + ((forall a b. C b => a->a) -> Int ) + +with a UserTypeCtxt of (FunSigCtxt "f"). Then we'll do the co/contra thing +on the argument type of the (->) -- and at that point we want to switch +to a UserTypeCtxt of GenSigCtxt. Why? + +* Error messages. If we stick with FunSigCtxt we get errors like + * Could not deduce: C b + from the context: C b0 + bound by the type signature for: + f :: forall a b. C b => a->a + But of course f does not have that type signature! + Example tests: T10508, T7220a, Simple14 + +* Implications. We may decide to build an implication for the whole + ambiguity check, but we don't need one for each level within it, + and TcUnify.alwaysBuildImplication checks the UserTypeCtxt. + See Note [When to build an implication] +-} + ----------------- -- needs both un-type-checked (for origins) and type-checked (for wrapping) -- expressions |