https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93147
Bug ID: 93147
Summary: std::tuple of empty structs with member equality
operators has ambiguous equality operator
Product: gcc
Version: 9.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: kholdstare0.0 at gmail dot com
Target Milestone: ---
The following code:
#include <tuple>
struct A {
bool operator == (A) const { return true; }
};
struct B {
bool operator == (B) const { return true; }
};
using Tuple = std::tuple<A, B>;
bool example(Tuple a, Tuple b)
{
return a == b;
}
Results in an error:
<source>: In function 'bool example(Tuple, Tuple)':
<source>:18:21: error: request for member 'operator==' is ambiguous
18 | return a == b;
| ^
<source>:4:14: note: candidates are: 'bool A::operator==(A) const'
4 | bool operator == (A) const { return true; }
| ^~~~~~~~
<source>:8:14: note: 'bool B::operator==(B) const'
8 | bool operator == (B) const { return true; }
| ^~~~~~~~
Compiler returned: 1
Here is the godbolt link, with the preprocessor output on the side:
https://godbolt.org/z/a4TuB3
Some things to note:
- This compiles fine with Clang
- If the structs are non-empty, this also compiles fine
- If the comparison operators are non-members this also compiles fine
- The failure is exactly the same going back all the way to gcc 4
My suspicion is that tuple indirectly inherits the types `A` and `B`, and even
though it may be private inheritance, the compiler still finds both comparison
operators and considers them ambiguous. GCC tries to implicitly convert
`std::tuple<A, B>` into `A` and `B` to call `operator ==`.
