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.

Because in S constructor you'll get call to T constructor which could throw any exception.Clang is right, I believe — Severin Pappadeux, Apr 07 '15 at 03:28
@SeverinPappadeux that's true about the exceptions but the issue seems to be whether the code should be rejected immediately, or whether the effect of `= default` should be to *define as deleted* which g++ seems to be doing. — M.M, Apr 07 '15 at 03:32

T.C. · Accepted Answer · 2015-04-07T07:59:37.640

23

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.

edited Apr 07 '15 at 07:59

answered Apr 07 '15 at 03:32

T.C.

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Which version appeared in published C++14 ? (clang 3.7 still rejects the code with -std=c++14) – M.M Apr 07 '15 at 03:35
@MattMcNabb The issue has "C++14" status, so the second version should be in C++14. Note that this is a DR, so it probably will be implemented even in C++11 mode. – T.C. Apr 07 '15 at 03:36
@MattMcNabb 4296 is pretty much what was voted as C++14 modulo typos and such – Severin Pappadeux Apr 07 '15 at 03:37
@SeverinPappadeux Nah, 4296 has some C++1z stuff. I probably should quote 3936 or 4140. – T.C. Apr 07 '15 at 03:37
@T.C. Thank you, didn't know it. I thought 4296 is it because official standard was marked by December 15 (?) 2014 – Severin Pappadeux Apr 07 '15 at 03:40
@SeverinPappadeux Completing the ballot on the draft and then working through all the bureaucratic paperwork takes time :) The C++14 draft was voted out of the committee in February 2014. – T.C. Apr 07 '15 at 08:01
It seems that CWG1778 was reverted again for c++20: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1286r2.html – craffael Jun 10 '20 at 20:35

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

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