Program with "noexcept" constructor accepted by gcc, rejected by clang

Question

The code:

struct T { T() {} };

struct S
{
    T t;

    S() noexcept = default;
};

int main()
{
//    S s;
}

g++ 4.9.2 accepts this with no errors or warnings, however clang 3.6 and 3.7 report for line 7:

error: exception specification of explicitly defaulted default constructor does not match the calculated one

However, if the line S s; is not commented out, g++ 4.9.2 now reports:

noex.cc: In function 'int main()':
noex.cc:12:7: error: use of deleted function 'S::S()'
     S s;
       ^
noex.cc:7:5: note: 'S::S() noexcept' is implicitly deleted because its  exception-specification does not match the implicit exception-specification ''
     S() noexcept = default;
     ^

Which compiler is right for the original code?

---

Background:

g++ even allows the following to be added to main:

std::cout << std::is_constructible<S>::value << '\n';

which outputs 0. I encountered this problem when using clang to compile some complicated code that made heavy use of templates, SFINAE and noexcept. In that code S and T are template classes; so the behaviour depends on which types S was instantiated with. Clang rejects it with this error for some types, whereas g++ permits it and the SFINAE works based on is_constructible and similar traits.

Answer 1

Depends on the version of the standard you are consulting.

N3337 [dcl.fct.def.default]/p2:

An explicitly-defaulted function [...] may have an explicit exception-specification only if it is compatible (15.4) with the exception-specification on the implicit declaration.

which renders your original code ill-formed.

This was changed by CWG issue 1778 to read (N4296 [dcl.fct.def.default]/p3):

If a function that is explicitly defaulted is declared with an exception-specification that is not compatible (15.4) with the exception specification on the implicit declaration, then

• if the function is explicitly defaulted on its first declaration, it is defined as deleted;
• otherwise, the program is ill-formed.

which means that the constructor is now merely defined as deleted. (The above wording incorporated changes made by N4285, a post-C++14 paper making some cleanup changes intended to be purely editorial. The N3936 version is substantively the same.)

Presumably GCC implements CWG1778's resolution, while Clang doesn't.