Oleg Grenrus pushed to branch wip/T26295 at Glasgow Haskell Compiler / GHC Commits: a05f102c by Oleg Grenrus at 2025-08-11T09:22:39+00:00 Apply 2 suggestion(s) to 2 file(s) Co-authored-by: sheaf <sam.derbyshire@gmail.com> - - - - - 2 changed files: - compiler/GHC/Tc/Validity.hs - testsuite/tests/overloadedrecflds/should_run/T26295.hs Changes: ===================================== compiler/GHC/Tc/Validity.hs ===================================== @@ -1723,8 +1723,11 @@ checkHasFieldInst cls tys@[_k_ty, _r_rep, _a_rep, lbl_ty, r_ty, _a_ty] = | let lbl_str = FieldLabelString lbl , Just fl <- lookupTyConFieldLabel lbl_str tc -> do - -- this logic vaguely mirrors 'matchHasField', - -- generally we should allow to define HasField instances which GHC will not solve for. + -- GHC does not provide HasField instances for naughty record selectors + -- (see Note [Naughty record selectors] in GHC.Tc.TyCl.Utils), + -- so don't prevent the user from writing such instances. + -- See GHC.Tc.Instance.Class.matchHasField. + -- Test case: T26295. sel_id <- tcLookupId $ flSelector fl if isNaughtyRecordSelector sel_id then return () ===================================== testsuite/tests/overloadedrecflds/should_run/T26295.hs ===================================== @@ -12,7 +12,7 @@ data R = forall a b. (a ~ Int, b ~ Char) => MkR , field_b :: b } --- fields in R are naught, so we can define own HasField instances for them. +-- fields in R are naughty, so we can define custom HasField instancs for them instance a ~ Int => HasField "field_a" R a where getField (MkR a _) = a View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/a05f102c5e5fcdb99457914c44224993... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/a05f102c5e5fcdb99457914c44224993... You're receiving this email because of your account on gitlab.haskell.org.