diff options
Diffstat (limited to 'compiler/GHC/Tc/Utils/Unify.hs')
| -rw-r--r-- | compiler/GHC/Tc/Utils/Unify.hs | 15 |
1 files changed, 10 insertions, 5 deletions
diff --git a/compiler/GHC/Tc/Utils/Unify.hs b/compiler/GHC/Tc/Utils/Unify.hs index c1202f02d7..b8e946ac11 100644 --- a/compiler/GHC/Tc/Utils/Unify.hs +++ b/compiler/GHC/Tc/Utils/Unify.hs @@ -927,13 +927,18 @@ checkTvConstraints skol_info skol_tvs thing_inside emitResidualTvConstraint :: SkolemInfo -> [TcTyVar] -> TcLevel -> WantedConstraints -> TcM () emitResidualTvConstraint skol_info skol_tvs tclvl wanted - | isEmptyWC wanted - = return () - - | otherwise - = do { implic <- buildTvImplication skol_info skol_tvs tclvl wanted + | not (isEmptyWC wanted) || + checkTelescopeSkol skol_info + = -- checkTelescopeSkol: in this case, /always/ emit this implication + -- even if 'wanted' is empty. We need the implication so that we check + -- for a bad telescope. See Note [Skolem escape and forall-types] in + -- GHC.Tc.Gen.HsType + do { implic <- buildTvImplication skol_info skol_tvs tclvl wanted ; emitImplication implic } + | otherwise -- Empty 'wanted', emit nothing + = return () + buildTvImplication :: SkolemInfo -> [TcTyVar] -> TcLevel -> WantedConstraints -> TcM Implication buildTvImplication skol_info skol_tvs tclvl wanted |