diff options
Diffstat (limited to 'compiler')
| -rw-r--r-- | compiler/GHC/Hs/Types.hs | 32 |
1 files changed, 18 insertions, 14 deletions
diff --git a/compiler/GHC/Hs/Types.hs b/compiler/GHC/Hs/Types.hs index 78d03c5859..c250ec013e 100644 --- a/compiler/GHC/Hs/Types.hs +++ b/compiler/GHC/Hs/Types.hs @@ -92,7 +92,7 @@ import SrcLoc import Outputable import FastString import Maybes( isJust ) -import Util ( count, debugIsOn ) +import Util ( count ) import Data.Data hiding ( Fixity, Prefix, Infix ) @@ -965,9 +965,8 @@ hsWcScopedTvs sig_ty , HsIB { hsib_ext = vars , hsib_body = sig_ty2 } <- sig_ty1 = case sig_ty2 of - L _ (HsForAllTy { hst_fvf = vis_flag + L _ (HsForAllTy { hst_fvf = ForallInvis -- See Note [hsScopedTvs vis_flag] , hst_bndrs = tvs }) -> - ASSERT( vis_flag == ForallInvis ) -- See Note [hsScopedTvs vis_flag] vars ++ nwcs ++ hsLTyVarNames tvs _ -> nwcs hsWcScopedTvs (HsWC _ (XHsImplicitBndrs nec)) = noExtCon nec @@ -978,10 +977,9 @@ hsScopedTvs :: LHsSigType GhcRn -> [Name] hsScopedTvs sig_ty | HsIB { hsib_ext = vars , hsib_body = sig_ty2 } <- sig_ty - , L _ (HsForAllTy { hst_fvf = vis_flag + , L _ (HsForAllTy { hst_fvf = ForallInvis -- See Note [hsScopedTvs vis_flag] , hst_bndrs = tvs }) <- sig_ty2 - = ASSERT( vis_flag == ForallInvis ) -- See Note [hsScopedTvs vis_flag] - vars ++ hsLTyVarNames tvs + = vars ++ hsLTyVarNames tvs | otherwise = [] @@ -1027,17 +1025,23 @@ The conclusion of these discussions can be summarized as follows: > vfn :: forall x y -> tau(x,y) > vfn x y = \a b -> ... -- bad! -At the moment, GHC does not support visible 'forall' in terms, so we simply cement -our assumptions with an assert: +We cement this design by pattern-matching on ForallInvis in hsScopedTvs: - hsScopedTvs (HsForAllTy { hst_fvf = vis_flag, ... }) = - ASSERT( vis_flag == ForallInvis ) - ... + hsScopedTvs (HsForAllTy { hst_fvf = ForallInvis, ... }) = ... -In the future, this assert can be safely turned into a pattern match to support -visible forall in terms: +At the moment, GHC does not support visible 'forall' in terms. Nevertheless, +it is still possible to write erroneous programs that use visible 'forall's in +terms, such as this example: - hsScopedTvs (HsForAllTy { hst_fvf = ForallInvis, ... }) = ... + x :: forall a -> a -> a + x = x + +If we do not pattern-match on ForallInvis in hsScopedTvs, then `a` would +erroneously be brought into scope over the body of `x` when renaming it. +Although the typechecker would later reject this (see `TcValidity.vdqAllowed`), +it is still possible for this to wreak havoc in the renamer before it gets to +that point (see #17687 for an example of this). +Bottom line: nip problems in the bud by matching on ForallInvis from the start. -} --------------------- |