https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63604
Bug ID: 63604
Summary: [C++11] A direct-initialization of a reference should
use explicit conversion functions
Product: gcc
Version: 5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: kariya_mitsuru at hotmail dot com
The sample code below should be compiled successfully but it causes compilation
error by gcc.
==========================================
struct T {};
struct S {
explicit operator T() { return T(); }
};
int main()
{
S s;
T&& t(s);
(void) t;
}
==========================================
cf. http://melpon.org/wandbox/permlink/LHgajpAXzqTbpYDc
An initialization of a reference in a direct-initialization context should use
an explicit conversion function that converts to a class prvalue.
The latest C++ standard (n4140) 13.3.1.6 [over.match.ref]/p.1.1 says that
The conversion functions of S and its base classes are considered. Those
non-explicit conversion functions that are not hidden within S and yield type
“lvalue reference to cv2 T2” (when initializing an lvalue reference or an
rvalue reference to function) or “cv2 T2” or “rvalue reference to cv2 T2” (when
initializing an rvalue reference or an lvalue reference to function), where
“cv1 T” is reference-compatible (8.5.3) with “cv2 T2”, are candidate functions.
For direct-initialization, those explicit conversion functions that are not
hidden within S and yield type “lvalue reference to cv2 T2” or “cv2 T2” or
“rvalue reference to cv2 T2”, respectively, where T2 is same type as T or can
be converted to type T with a qualification conversion (4.4), are also
candidate functions.
I think that this sample code corresponds to the case “For
direct-initialization, ...”.
Note that this sample code is compiled successfully if the conversion function
returns an rvalue reference.
(cf. http://melpon.org/wandbox/permlink/kGpALX7zvzHzi7K5)
See also BUG 48453.
