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. Jason