On Thu, Jul 20, 2023 at 10:42:29AM -0400, Jason Merrill wrote:
> On 7/20/23 05:35, Nathaniel Shead wrote:
> > This adds rudimentary lifetime tracking in C++ constexpr contexts,
> > allowing the compiler to report errors with using values after their
> > backing has gone out of scope. We don't yet handle other ways of
> > accessing values outside their lifetime (e.g. following explicit
> > destructor calls).
>
> Incidentally, much of that should be straightforward to handle by no longer
> ignoring clobbers here:
>
> > case MODIFY_EXPR:
> > if (cxx_dialect < cxx14)
> > goto fail;
> > if (!RECUR (TREE_OPERAND (t, 0), any))
> > return false;
> > /* Just ignore clobbers. */
> > if (TREE_CLOBBER_P (TREE_OPERAND (t, 1)))
> > return true;
>
> Assignment from a clobber represents end of lifetime to the middle-end. This
> can be a follow-up patch.
Thanks, this is very helpful to know. I'll keep this in mind.
> > @@ -7051,10 +7065,17 @@ cxx_eval_constant_expression (const constexpr_ctx
> > *ctx, tree t,
> > return ctx->ctor;
> > if (VAR_P (t))
> > if (tree v = ctx->global->get_value (t))
> > - {
> > - r = v;
> > - break;
> > - }
> > + {
> > + r = v;
> > + break;
> > + }
> > + if (ctx->global->is_outside_lifetime (t))
> > + {
> > + if (!ctx->quiet)
> > + outside_lifetime_error (loc, t);
> > + *non_constant_p = true;
> > + break;
> > + }
>
> Shouldn't this new check also be under the if (VAR_P (t))? A CONST_DECL
> can't go out of scope.
>
> Jason
>
Yup you're right; I didn't properly read the documentation on what a
CONST_DECL was and misunderstood. I'll fix this up for the next version.
