diff options
Diffstat (limited to 'compiler/deSugar/Check.hs')
| -rw-r--r-- | compiler/deSugar/Check.hs | 5 |
1 files changed, 4 insertions, 1 deletions
diff --git a/compiler/deSugar/Check.hs b/compiler/deSugar/Check.hs index 545aacef51..7a5f889423 100644 --- a/compiler/deSugar/Check.hs +++ b/compiler/deSugar/Check.hs @@ -790,10 +790,13 @@ translatePat fam_insts pat = case pat of -- overloaded list ListPat (ListPatTc elem_ty (Just (pat_ty, _to_list))) lpats | Just e_ty <- splitListTyConApp_maybe pat_ty + , (_, norm_e_ty) <- normaliseType fam_insts Nominal e_ty + -- e_ty can be a type family instance, like + -- `It (List a)`, but we prefer `a`, see Trac #14547 , (_, norm_elem_ty) <- normaliseType fam_insts Nominal elem_ty -- elem_ty is frequently something like -- `Item [Int]`, but we prefer `Int` - , norm_elem_ty `eqType` e_ty -> + , norm_elem_ty `eqType` norm_e_ty -> -- We have to ensure that the element types are exactly the same. -- Otherwise, one may give an instance IsList [Int] (more specific than -- the default IsList [a]) with a different implementation for `toList' |