diff options
Diffstat (limited to 'compiler/typecheck/TcExpr.hs')
| -rw-r--r-- | compiler/typecheck/TcExpr.hs | 48 |
1 files changed, 46 insertions, 2 deletions
diff --git a/compiler/typecheck/TcExpr.hs b/compiler/typecheck/TcExpr.hs index df735d23d6..1387e89089 100644 --- a/compiler/typecheck/TcExpr.hs +++ b/compiler/typecheck/TcExpr.hs @@ -1104,6 +1104,10 @@ isHsValArg (HsValArg {}) = True isHsValArg (HsTypeArg {}) = False isHsValArg (HsArgPar {}) = False +isHsTypeArg :: HsArg tm ty -> Bool +isHsTypeArg (HsTypeArg {}) = True +isHsTypeArg _ = False + isArgPar :: HsArg tm ty -> Bool isArgPar (HsArgPar {}) = True isArgPar (HsValArg {}) = False @@ -1283,6 +1287,14 @@ tcArgs :: LHsExpr GhcRn -- ^ The function itself (for err msgs only) -> TcM (HsWrapper, [LHsExprArgOut], TcSigmaType) -- ^ (a wrapper for the function, the tc'd args, result type) tcArgs fun orig_fun_ty fun_orig orig_args herald + | fun_is_out_of_scope + , any isHsTypeArg orig_args + = failM -- See Note [VTA for out-of-scope functions] + -- We have /already/ emitted a CHoleCan constraint (in tcInferFun), + -- which will later cough up a "Variable not in scope error", so + -- we can simply fail now, avoiding a confusing error cascade + + | otherwise = go [] 1 orig_fun_ty orig_args where -- Don't count visible type arguments when determining how many arguments @@ -1291,6 +1303,11 @@ tcArgs fun orig_fun_ty fun_orig orig_args herald -- See Note [Herald for matchExpectedFunTys] in TcUnify. orig_expr_args_arity = count isHsValArg orig_args + fun_is_out_of_scope -- See Note [VTA for out-of-scope functions] + = case fun of + L _ (HsUnboundVar {}) -> True + _ -> False + go _ _ fun_ty [] = return (idHsWrapper, [], fun_ty) go acc_args n fun_ty (HsArgPar sp : args) @@ -1374,6 +1391,33 @@ GHCs we had an ASSERT that Required could not occur here. The ice is thin; c.f. Note [No Required TyCoBinder in terms] in TyCoRep. +Note [VTA for out-of-scope functions] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Suppose 'wurble' is not in scope, and we have + (wurble @Int @Bool True 'x') + +Then the renamer will make (HsUnboundVar "wurble) for 'wurble', +and the typechecker will typecheck it with tcUnboundId, giving it +a type 'alpha', and emitting a deferred CHoleCan constraint, to +be reported later. + +But then comes the visible type application. If we do nothing, we'll +generate an immediate failure (in tc_app_err), saying that a function +of type 'alpha' can't be applied to Bool. That's insane! And indeed +users complain bitterly (#13834, #17150.) + +The right error is the CHoleCan, which reports 'wurble' as out of +scope, and tries to give its type. + +Fortunately in tcArgs we still have acces to the function, so +we can check if it is a HsUnboundVar. If so, we simply fail +immediately. We've already inferred the type of the function, +so we'll /already/ have emitted a CHoleCan constraint; failing +preserves that constraint. + +A mild shortcoming of this approach is that we thereby +don't typecheck any of the arguments, but so be it. + Note [Visible type application zonk] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * Substitutions should be kind-preserving, so we need kind(tv) = kind(ty_arg). @@ -1845,8 +1889,8 @@ tcUnboundId rn_expr unbound res_ty ; let ev = mkLocalId name ty ; can <- newHoleCt (ExprHole unbound) ev ty ; emitInsoluble can - ; tcWrapResultO (UnboundOccurrenceOf occ) rn_expr (HsVar noExtField (noLoc ev)) - ty res_ty } + ; tcWrapResultO (UnboundOccurrenceOf occ) rn_expr + (HsVar noExtField (noLoc ev)) ty res_ty } {- |