https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115699
Bug ID: 115699
Summary: Anonymous structs should compare member-wise with
C++20 defaulted operator==
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: mital at mitalashok dot co.uk
Target Milestone: ---
Consider:
```
struct A {
struct {
int i;
};
friend bool operator==(A, A) = default;
};
static_assert(A{} == A{});
```
This currently fails as the operator== is marked as deleted since it tries to
compare the anonymous struct members as a whole:
```
<source>:7:24: error: use of deleted function 'constexpr bool operator==(A, A)'
7 | static_assert(A{} == A{});
| ^
<source>:5:15: note: 'constexpr bool operator==(A, A)' is implicitly deleted
because the default definition would be ill-formed:
5 | friend bool operator==(A, A) = default;
| ^~~~~~~~
<source>:2:3: error: no match for 'operator==' (operand types are 'A::<unnamed
struct>' and 'A::<unnamed struct>')
2 | struct {
| ^~~~~~
<source>:5:15: note: candidate: 'constexpr bool operator==(A, A)' (deleted)
5 | friend bool operator==(A, A) = default;
| ^~~~~~~~
<source>:5:26: note: no known conversion for argument 1 from 'A::<unnamed
struct>' to 'A'
5 | friend bool operator==(A, A) = default;
| ^
```
This is a GCC/C11 extension so there is no "official" expected behaviour, but
this is surprising. This also leads to overload resolution being done with the
unnamed struct type:
```
struct X {
struct {
bool b;
};
bool c;
template<typename T>
friend constexpr bool operator==(T& L, T& R) {
// T is the type of the anonymous struct
static_assert(sizeof(T) == sizeof(bool));
return L.b == R.b;
}
friend constexpr bool operator==(const X& L, const X& R) = default;
};
static_assert(X{} == X{});
```
Where it might make more sense for each member of the anonymous struct to be
considered a member of the class the struct is declared in so when operator==
compares member by member, it compares each member of the anonymous struct.
(This also applies to defaulted operator<=>)
