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Subject: Re: static_assert: substitution failure *IS* an error?
From: Brian Bi (bbi5291_at_[hidden])
Date: 2021-01-14 19:06:54

It sounds like the real problem here is that you have a situation where:

   - there are overloaded function templates, and
   - a hard error will occur if you attempt to instantiate one of their
   signatures, and
   - this problem can be avoided by simply not specifying a template
   argument list (so deduction will fail and the substitution step that causes
   the hard error will not occur), but for some reason you do need to
   explicitly specify a template argument list in order to call the desired

If this is your situation, then I guess you are out of luck, but it's hard
for me to imagine how you end up in this situation in the first place. (Why
would it be a good idea to have two overloads where one of them takes
std::shared_ptr and the other takes std::atomic?)

On Thu, Jan 14, 2021 at 4:06 PM Ville Voutilainen via Std-Discussion <
std-discussion_at_[hidden]> wrote:

> On Thu, 14 Jan 2021 at 22:19, Matthew Woehlke via Std-Discussion
> <std-discussion_at_[hidden]> wrote:
> >
> > Inspired by the combination of the recent thread on static_assert in
> > constexpr and a problem I ran into with real code... Consider the
> following:
> >
> > template <typename T>
> > void foo(volatile std::atomic<T>*,
> > typename std::atomic<T>::value_type)
> > {}
> >
> > template <typename T>
> > void foo(std::shared_ptr<T>*, std::shared_ptr<T>)
> > {}
> >
> > std::shared_ptr<std::vector<int>> a;
> > foo<std::vector<int>>(&a, a);
> >
> > (Live demo: https://godbolt.org/z/r6GMhY)
> >
> > It *looks* like this code ought to compile... and indeed, it compiles
> > *if* the template parameter is removed from the invocation of `foo`.
> > Without it, however, the static_assert that is tripped trying to
> > evaluate std::atomic<T>::value_type for the first overload halts
> > compilation, even though the first overload isn't viable *anyway*.
> >
> > This... seems undesirable, and contrary to SFINAE. Is this intended?
> It's not contrary to SFINAE. SFINAE applies in an immediate context,
> and a static_assert cannot appear
> in an immediate context. The problem here is, as far as I understand
> it, that the call with an explicit
> template argument triggers instantiation, and that triggers the
> static_assert.
> --
> Std-Discussion mailing list
> Std-Discussion_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-discussion

*Brian Bi*

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