diff options
Diffstat (limited to 'compiler/GHC/Tc/Instance/Class.hs')
| -rw-r--r-- | compiler/GHC/Tc/Instance/Class.hs | 20 |
1 files changed, 18 insertions, 2 deletions
diff --git a/compiler/GHC/Tc/Instance/Class.hs b/compiler/GHC/Tc/Instance/Class.hs index d15777cc5f..8269c67ce5 100644 --- a/compiler/GHC/Tc/Instance/Class.hs +++ b/compiler/GHC/Tc/Instance/Class.hs @@ -30,7 +30,7 @@ import GHC.Builtin.Types import GHC.Builtin.Types.Prim( eqPrimTyCon, eqReprPrimTyCon ) import GHC.Builtin.Names -import GHC.Types.Name.Reader( lookupGRE_FieldLabel ) +import GHC.Types.Name.Reader( lookupGRE_FieldLabel, greMangledName ) import GHC.Types.SafeHaskell import GHC.Types.Name ( Name, pprDefinedAt ) import GHC.Types.Var.Env ( VarEnv ) @@ -659,6 +659,20 @@ may be solved by a user-supplied HasField instance. Similarly, if we encounter a HasField constraint where the field is not a literal string, or does not belong to the type, then we fall back on the normal constraint solver behaviour. + + +Note [Unused name reporting and HasField] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +When a HasField constraint is solved by the type-checker, we must record a use +of the corresponding field name, as otherwise it might be reported as unused. +See #19213. We need to call keepAlive to add the name to the tcg_keep set, +which accumulates names used by the constraint solver, as described by +Note [Tracking unused binding and imports] in GHC.Tc.Types. + +We need to call addUsedGRE as well because there may be a deprecation warning on +the field, which will be reported by addUsedGRE. But calling addUsedGRE without +keepAlive is not enough, because the field might be defined locally, and +addUsedGRE extends tcg_used_gres with imported GREs only. -} -- See Note [HasField instances] @@ -708,7 +722,9 @@ matchHasField dflags short_cut clas tys -- cannot have an existentially quantified type), and -- it must not be higher-rank. ; if not (isNaughtyRecordSelector sel_id) && isTauTy sel_ty - then do { addUsedGRE True gre + then do { -- See Note [Unused name reporting and HasField] + addUsedGRE True gre + ; keepAlive (greMangledName gre) ; return OneInst { cir_new_theta = theta , cir_mk_ev = mk_ev , cir_what = BuiltinInstance } } |