Fix newtype deriving properly (un-doing Audreys patch)

[ghc-hetmet.git] / compiler / typecheck / TcInstDcls.lhs

diff --git a/compiler/typecheck/TcInstDcls.lhs b/compiler/typecheck/TcInstDcls.lhs
index 32b51d0..1be9ffc 100644 (file)
--- a/compiler/typecheck/TcInstDcls.lhs
+++ b/compiler/typecheck/TcInstDcls.lhs
@@ -571,6 +571,9 @@ tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = NewTypeDerived mb_preds })
                   the_rhs = mkHsConApp cls_data_con cls_inst_tys $
                             map HsVar (sc_dict_ids ++ op_ids)
 
+               -- Warning: this HsCase scrutinises a value with a PredTy, which is
+               --          never otherwise seen in Haskell source code. It'd be
+               --          nicer to generate Core directly!
             ; return (HsCase (noLoc coerced_rep_dict) $
                       MatchGroup [the_match] (mkFunTy inst_head_ty inst_head_ty)) }
        where