diff options
Diffstat (limited to 'compiler/GHC/Tc/TyCl/Instance.hs')
| -rw-r--r-- | compiler/GHC/Tc/TyCl/Instance.hs | 15 |
1 files changed, 12 insertions, 3 deletions
diff --git a/compiler/GHC/Tc/TyCl/Instance.hs b/compiler/GHC/Tc/TyCl/Instance.hs index b0dfa80f90..4cc8a79e1e 100644 --- a/compiler/GHC/Tc/TyCl/Instance.hs +++ b/compiler/GHC/Tc/TyCl/Instance.hs @@ -512,9 +512,18 @@ tcClsInstDecl (L loc (ClsInstDecl { cid_poly_ty = hs_ty, cid_binds = binds -- Check for missing associated types and build them -- from their defaults (if available) + ; is_boot <- tcIsHsBootOrSig + ; let atItems = classATItems clas ; tf_insts2 <- mapM (tcATDefault loc mini_subst defined_ats) - (classATItems clas) - + (if is_boot then [] else atItems) + -- Don't default type family instances, but rather omit, in hsig/hs-boot. + -- Since hsig/hs-boot files are essentially large binders we want omission + -- of the definition to result in no restriction, rather than for example + -- attempting to "pattern match" with the invisible defaults and generate + -- equalities. Without further handling, this would just result in a panic + -- anyway. + -- See https://github.com/ghc-proposals/ghc-proposals/pull/320 for + -- additional discussion. ; return (df_stuff, tf_insts1 ++ concat tf_insts2) } @@ -539,8 +548,8 @@ tcClsInstDecl (L loc (ClsInstDecl { cid_poly_ty = hs_ty, cid_binds = binds all_insts = tyfam_insts ++ datafam_insts -- In hs-boot files there should be no bindings - ; is_boot <- tcIsHsBootOrSig ; let no_binds = isEmptyLHsBinds binds && null uprags + ; is_boot <- tcIsHsBootOrSig ; failIfTc (is_boot && not no_binds) badBootDeclErr ; return ( [inst_info], all_insts, deriv_infos ) } |