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From: "acoplan at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/60512] would be useful if gcc implemented __has_feature
 similary to clang
Date: Tue, 11 Apr 2023 16:39:48 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D60512

--- Comment #13 from Alex Coplan <acoplan at gcc dot gnu.org> ---
Clang recognizes the "cxx_defaulted_functions" feature to detect whether "=
=3D
default" functions are supported.

It's clear that __has_feature (cxx_defaulted_functions) should evaluate to 1
for -std=3Dc++11 and above. It's less clear whether GCC intends to support =
"=3D
default" functions in C++98 mode as an extension, and therefore whether
__has_extension (cxx_defaulted_functions) should evaluate to 1 with -std=3D=
c++98.

I noticed that e.g. we accept:

struct S {
    int x;
    S(int a) : x(a) {}
    S() =3D default;
};
void f() {
    S s;
}

with -std=3Dc++98 and emit a -Wc++11-extensions warning. This suggests that=
 "=3D
default" might be supported as an extension, but it's not clear. By contras=
t,
it seems that deleted functions (detected with "cxx_deleted_functions") are=
n't
supported in C++98 mode, since e.g.

struct S {
    S() =3D delete;
};
void f() {
    S s;
}

doesn't get rejected with -std=3Dc++98. It would be good to get some input =
from
C++ maintainers on the cxx_defaulted_functions case.=