https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96299
Bug ID: 96299
Summary: Defaulted operator <=> implicitly deleted when a
member has operator < and operator == and return type
is specified
Product: gcc
Version: 10.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: gcc-bugzilla at mysko dot org
Target Milestone: ---
The following sample fails to compile with GCC 10.1 and with trunk as of
2020-07-22.
===================
#include <compare>
struct Legacy
{
bool operator==(Legacy const&) const;
bool operator<(Legacy const&) const;
};
struct A
{
std::strong_ordering operator <=> (A const& other) const = default;
Legacy l;
};
int main()
{
A a, b;
return a < b;
}
===================
With the following error:
<source>: In function 'int main()':
<source>:19:14: error: use of deleted function 'constexpr std::strong_ordering
A::operator<=>(const A&) const'
19 | return a < b;
| ^
<source>:11:24: note: 'constexpr std::strong_ordering A::operator<=>(const A&)
const' is implicitly deleted because the default definition would be
ill-formed:
11 | std::strong_ordering operator <=> (A const& other) const = default;
| ^~~~~~~~
<source>:11:24: error: no match for 'operator<=>' (operand types are 'Legacy'
and 'Legacy')
>From my understanding of p1186r3
(http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1186r3.html) the code
should be accepted. The GCC standards support table lists p1186r3 as
implemented in GCC 10. According to p1186r3, the defaulted operator <=> should
use the member's < and == operators if the return type is specified
(strong/weak/partial), and should not require a <=> operator for the member.
