On Thu, Feb 08, 2024 at 04:53:45PM -0500, Jason Merrill wrote:
> On 2/8/24 11:51, Marek Polacek wrote:
> > On Thu, Feb 08, 2024 at 08:49:28AM -0500, Patrick Palka wrote:
> > > On Wed, 7 Feb 2024, Marek Polacek wrote:
> > >
> > > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > > >
> > > > -- >8 --
> > > > Here the problem is that we give hard errors while substituting
> > > > template parameters during overload resolution of is_throwable
> > > > which has an invalid throw in decltype.
> > > >
> > > > The backtrace shows that fn_type_unification -> instantiate_template
> > > > -> tsubst* passes complain=0 as expected, but build_throw doesn't
> > > > have a complain parameter. So let's add one. Also remove a redundant
> > > > local variable which I should have removed in my P2266 patch.
> > > >
> > > > But there's still something not quite clear to me. I claim that 'b'
> > > > in the testcase should evaluate to false since the first overload ought
> > > > to have been discarded. EDG 6.6 agrees, but clang++, msvc, and icx
> > > > evaluate
> > > > it to true. Who's right?
>
> I think it should be true since P1155, which we implement in C++20 mode and
> above (or rather, we implement the sequel P2266); since then we implicitly
> move from the function parameter.
>
> The patch looks good except that we should test this expected value.
I could add
#if __cplusplus >= 202002L
static_assert (b, "move from the function parameter");
#else
static_assert (!b, "no move from the function parameter");
#endif
but that's going to fail for C++20 and above. I wonder if this is the
second half of the problem in 113789?
I could comment the first static_assert and add a FIXME if that sounds good?
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp0x/sfinae69.C
> > @@ -0,0 +1,16 @@
> > +// PR c++/98388
> > +// { dg-do compile { target c++11 } }
> > +
> > +struct moveonly {
> > + moveonly() = default;
> > + moveonly(moveonly&&) = default;
> > +};
> > +
> > +template<class T>
> > +constexpr auto is_throwable(T t) -> decltype(throw t, true) {
> > + return true;
> > +}
> > +template<class T>
> > +constexpr bool is_throwable(...) { return false; }
> > +
> > +constexpr bool b = is_throwable<moveonly>(moveonly{});
Marek
