On Mon, Feb 5, 2018 at 1:45 PM, Jason Merrill <ja...@redhat.com> wrote: > On Mon, Feb 5, 2018 at 8:37 AM, Marek Polacek <pola...@redhat.com> wrote: >> On Fri, Feb 02, 2018 at 02:11:27PM -0500, Jason Merrill wrote: >>> On Thu, Jan 25, 2018 at 4:16 PM, Marek Polacek <pola...@redhat.com> wrote: >>> > This is a similar problem to 83116: we'd cached a constexpr call, but >>> > after a >>> > store the result had become invalid, yet we used the wrong result again >>> > when >>> > encountering the same call later. This resulted in evaluating a >>> > THROW_EXPR >>> > which doesn't work. Details in >>> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83692#c5 >>> > >>> > The fix for 83116 didn't work here, because when evaluating the body of >>> > the >>> > ctor via store_init_value -> cxx_constant_value we are in STRICT, so we do >>> > cache. >>> >>> > It seems that we may no longer rely on the constexpr call table when we >>> > do cxx_eval_store_expression, because that just rewrites *valp, i.e. the >>> > value of an object. Might be too big a hammer again, but I couldn't think >>> > of how I could guard the caching of a constexpr call. >>> >>> > This doesn't manifest in C++14 because build_special_member_call in C++17 >>> > is >>> > more aggressive with copy elisions (as required by P0135 which changed >>> > how we >>> > view prvalues). In C++14 build_special_member_call produces a CALL_EXPR, >>> > so >>> > expand_default_init calls maybe_constant_init, for which STRICT is false, >>> > so >>> > we avoid caching as per 83116. >>> >>> So it sounds like the problem is using cxx_constant_value for the >>> diagnostic when it has different semantics from the >>> maybe_constant_init that follows right after. I guess we want a >>> cxx_constant_init function that is a hybrid of the two. >> >> So like the following? Thanks, >> >> Bootstrapped/regtested on x86_64-linux, ok for trunk? >> >> 2018-02-04 Marek Polacek <pola...@redhat.com> >> >> PR c++/83692 >> * constexpr.c (cxx_constant_init): New function. >> * cp-tree.h (cxx_constant_init): Declare. >> * typeck2.c (store_init_value): Call cxx_constant_init instead of >> cxx_constant_value. >> >> +/* Like cxx_constant_value, but non-strict mode. */ >> + >> +tree >> +cxx_constant_init (tree t, tree decl) >> +{ >> + return cxx_eval_outermost_constant_expr (t, false, false, decl); >> +} > > Hmm, that doesn't do the TARGET_EXPR stripping that > maybe_constant_init does. I was thinking of a version of > maybe_constant_init that passes false to allow_non_constant. Probably > by making "maybe_constant_init" and cxx_constant_init both call the > current function with an additional parameter. And then the existing > call to maybe_constant_init can move under an 'else' to avoid > redundant constexpr evaluation.
Want me to take this over? Jason
