This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 114a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2024-04-18
Consider the following example:
template<class T>struct Y {
typedef typename T::value_type blah; // #1
void swap(Y<T> &);
};
template<class T>
void swap(Y<T>& Left, Y<T>& Right) noexcept(noexcept(Left.swap(Right))) { }
template <class T> struct Z {
void swap(Z<T> &);
};
template<class T>
void swap(Z<T>& Left, Z<T>& Right) noexcept(noexcept(Left.swap(Right))) { }
Z<int> x00, y00;
constexpr bool b00 = noexcept(x00.swap(y00));
template void swap<int>(Z<int>&, Z<int>&) noexcept(b00); // #2
The question here is whether the explicit instantiation of
swap<int>(Z<int>&, Z<int>&)
at #2 instantiates the exception specification of
swap<int>(Y<int>&, Y<int>&)
which would instantiate Y<int>, resulting in an error on the declaration of
typedef typename T::value_type blah;
at #1.
According to 13.9.2 [temp.inst] paragraph 14,
The noexcept-specifier of a function template specialization is not instantiated along with the function declaration; it is instantiated when needed (14.5 [except.spec]).
According to 14.5 [except.spec] bullet 13.3, one of the reasons an exception specification is needed is:
the exception specification is compared to that of another declaration (e.g., an explicit specialization or an overriding virtual function);
Such a comparison is presumably needed when determining which function template the explicit instantiation is referring to, making the program ill-formed. However, there is implementation variance on this point.
CWG 2022-11-10
There are related problems in this area; CWG is seeking input to form a holistic view.