https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99287
--- Comment #6 from Patrick Palka <ppalka at gcc dot gnu.org> --- (In reply to Jakub Jelinek from comment #4) > (In reply to Patrick Palka from comment #3) > > IIUC, those two types are actually the same, it's just that one of them was > > obtained through the char_type alias, and it seems debug_tree prefers to > > show the name of the alias when the type came from one. > > > > It seems the issue ultimately is in cxx_eval_increment_expression: during > > evaluation of ++__first with __first = &"mystr"[n] + m and with lval=false, > > since cxx_fold_pointer_plus_expression and not fold_build2 is responsible > > for simplifying this POINTER_PLUS_EXPR to &"mystr"[n+m], returning 'mod' > > actually returns the unreduced &"mystr"[n] + m rather than the reduced > > &"mystr"[n+m] that we obtain as part of constexpr evaluation of the > > temporary MODIFY_EXPR. This unreduced return value of cxx_eval_increment > > interfers with later constexpr evaluation, e.g. folding of the > > POINTER_DIFF_EXPR. > > > > I'm testing the following which updates 'mod' with the result of evaluation > > of the MODIFY_EXPR: > > > > --- a/gcc/cp/constexpr.c > > +++ b/gcc/cp/constexpr.c > > @@ -5582,20 +5582,14 @@ cxx_eval_increment_expression (const constexpr_ctx > > *ctx, tree t, > > /* Storing the modified value. */ > > tree store = build2_loc (cp_expr_loc_or_loc (t, input_location), > > MODIFY_EXPR, type, op, mod); > > - cxx_eval_constant_expression (ctx, store, > > - true, non_constant_p, overflow_p); > > + mod = cxx_eval_constant_expression (ctx, store, false, > > + non_constant_p, overflow_p); > > ggc_free (store); > > Maybe; I'm a little bit worried here though because > cxx_eval_constant_expression > can in various cases return the tree passed to it on failure (i.e. the > store) or can return NULL_TREE. > And store is then ggc_freed, so shouldn't be really used afterwards. > So perhaps > tree new_mod = cxx_eval_constant_expression (ctx, store, false, > non_constant_p, overflow_p); > if (new_mod && new_mod != store) > mod = new_mod; > > ggc_free (store); > ? Good point. Though I don't see how cxx_eval_constant_expression could plausibly return NULL_TREE here since we're dealing with a MODIFY_EXPR? And I think checking *non_constant_p could subsume checking new_mod != store. So perhaps just @@ -5582,20 +5582,16 @@ cxx_eval_increment_expression (const constexpr_ctx *ctx, tree t, /* Storing the modified value. */ tree store = build2_loc (cp_expr_loc_or_loc (t, input_location), MODIFY_EXPR, type, op, mod); - cxx_eval_constant_expression (ctx, store, - true, non_constant_p, overflow_p); + mod = cxx_eval_constant_expression (ctx, store, false, + non_constant_p, overflow_p); ggc_free (store); + if (*non_constant_p) + return t; would cover our bases. (In reply to Jakub Jelinek from comment #5) > Or perhaps another option would be instead of > return mod; > do > return cxx_eval_constant_expression (ctx, mod, false, non_constant_p, > overflow_p); > ? I think should work, though it'd mean we'd be redundantly evaluating 'mod' twice in the !lval case I suppose.