diff options
| -rw-r--r-- | compiler/types/FunDeps.lhs | 21 |
1 files changed, 19 insertions, 2 deletions
diff --git a/compiler/types/FunDeps.lhs b/compiler/types/FunDeps.lhs index 8f255b7227..77010de984 100644 --- a/compiler/types/FunDeps.lhs +++ b/compiler/types/FunDeps.lhs @@ -466,10 +466,11 @@ checkInstCoverage be_liberal clas theta inst_taus | if be_liberal then liberal_ok else conservative_ok = Nothing | otherwise - = Just msg + = pprTrace "cic" (vcat [ppr clas <+> ppr inst_taus, ppr fd, ppr ls_tvs, ppr rs_tvs, ppr (oclose theta ls_tvs), ppr theta]) $ + Just msg where (ls,rs) = instFD fd tyvars inst_taus - ls_tvs = tyVarsOfTypes ls + ls_tvs = closeOverKinds (tyVarsOfTypes ls) -- See Note [Closing over kinds in coverage] rs_tvs = tyVarsOfTypes rs conservative_ok = rs_tvs `subVarSet` ls_tvs @@ -492,6 +493,22 @@ checkInstCoverage be_liberal clas theta inst_taus ptext (sLit "Using UndecidableInstances might help") ] \end{code} +Note [Closing over kinds in coverage] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Suppose we have a fundep (a::k) -> b +Then if 'a' is instantiated to (x y), where x:k2->*, y:k2, +then fixing x really fixes k2 as well, and so k2 should be added to +the lhs tyvars in the fundep check. + +Example (Trac #8391), using liberal coverage + + type Foo a = a -- Foo :: forall k. k -> k + class Bar a b | a -> b + instance Bar a (Foo a) + +In the instance decl, (a:k) does fix (Foo k a), but only if we notice +that (a:k) fixes k. + Note [Coverage condition] ~~~~~~~~~~~~~~~~~~~~~~~~~ Example |