diff options
Diffstat (limited to 'compiler/GHC/Tc/Deriv/Infer.hs')
| -rw-r--r-- | compiler/GHC/Tc/Deriv/Infer.hs | 10 |
1 files changed, 3 insertions, 7 deletions
diff --git a/compiler/GHC/Tc/Deriv/Infer.hs b/compiler/GHC/Tc/Deriv/Infer.hs index 7276bfde83..2466931219 100644 --- a/compiler/GHC/Tc/Deriv/Infer.hs +++ b/compiler/GHC/Tc/Deriv/Infer.hs @@ -194,13 +194,9 @@ inferConstraintsStock (DerivInstTys { dit_cls_tys = cls_tys ] -- Stupid constraints from DatatypeContexts. Note that we -- must gather these constraints from the data constructors, - -- not from the parent type constructor, as the latter can - -- lead to redundant constraints in some cases. For example, - -- the derived Eq instance for: - -- - -- data Show a => T a = MkT deriving Eq - -- - -- Should not have Show in the instance context (#20501). + -- not from the parent type constructor, as the latter could + -- lead to redundant constraints due to thinning. + -- See Note [The stupid context] in GHC.Core.DataCon. stupid_theta = [ substTyWith (dataConUnivTyVars data_con) all_rep_tc_args |