diff options
Diffstat (limited to 'compiler/GHC/Tc/Solver.hs')
| -rw-r--r-- | compiler/GHC/Tc/Solver.hs | 47 |
1 files changed, 43 insertions, 4 deletions
diff --git a/compiler/GHC/Tc/Solver.hs b/compiler/GHC/Tc/Solver.hs index 991708111d..2d97b84e9d 100644 --- a/compiler/GHC/Tc/Solver.hs +++ b/compiler/GHC/Tc/Solver.hs @@ -218,7 +218,13 @@ simplifyAndEmitFlatConstraints wanted ; traceTc "emitFlatConstraints {" (ppr wanted) ; case floatKindEqualities wanted of Nothing -> do { traceTc "emitFlatConstraints } failing" (ppr wanted) - ; emitConstraints wanted -- So they get reported! + -- Emit the bad constraints, wrapped in an implication + -- See Note [Wrapping failing kind equalities] + ; tclvl <- TcM.getTcLevel + ; implic <- buildTvImplication UnkSkol [] tclvl wanted + -- UnkSkol: doesn't matter, because + -- we bind no skolem varaibles here + ; emitImplication implic ; failM } Just (simples, holes) -> do { _ <- promoteTyVarSet (tyCoVarsOfCts simples) @@ -376,6 +382,40 @@ All this is done: See also #18062, #11506 +Note [Wrapping failing kind equalities] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +In simplifyAndEmitFlatConstraints, if we fail to get down to simple +flat constraints we will +* re-emit the constraints so that they are reported +* fail in the monad +But there is a Terrible Danger that, if -fdefer-type-errors is on, and +we just re-emit an insoluble constraint like (* ~ (*->*)), that we'll +report only a warning and proceed with compilation. But if we ever fail +in the monad it should be fatal; we should report an error and stop after +the type checker. If not, chaos results: #19142. + +Our solution is this: +* Even with -fdefer-type-errors, inside an implication with no place for + value bindings (ic_binds = CoEvBindsVar), report failing equalities as + errors. We have to do this anyway; see GHC.Tc.Errors + Note [Failing equalities with no evidence bindings]. + +* Right here in simplifyAndEmitFlatConstraints, use buildTvImplication + to wrap the failing constraint in a degenerate implication (no + skolems, no theta, no bumped TcLevel), with ic_binds = CoEvBindsVar. + That way any failing equalities will lead to an error not a warning, + irrespective of -fdefer-type-errors. + + This is a slight hack, because the implication doesn't have a bumped + TcLevel, but that doesn't matter. + +We re-emit the implication rather than reporting the errors right now, +so that the error mesages are improved by other solving and defaulting. +e.g. we prefer + Cannot match 'Type->Type' with 'Type' +to Cannot match 'Type->Type' with 'TYPE r0' + + Note [floatKindEqualities vs approximateWC] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ floatKindEqualities and approximateWC are strikingly similar to each @@ -757,13 +797,12 @@ simplifyInteractive wanteds ------------------ simplifyDefault :: ThetaType -- Wanted; has no type variables in it - -> TcM () -- Succeeds if the constraint is soluble + -> TcM Bool -- Return if the constraint is soluble simplifyDefault theta = do { traceTc "simplifyDefault" empty ; wanteds <- newWanteds DefaultOrigin theta ; unsolved <- runTcSDeriveds (solveWantedsAndDrop (mkSimpleWC wanteds)) - ; reportAllUnsolved unsolved - ; return () } + ; return (isEmptyWC unsolved) } ------------------ tcCheckSatisfiability :: InertSet -> Bag EvVar -> TcM (Maybe InertSet) |