[clang] [libcxx] [SemaCXX] Implement CWG2137 (list-initialization from objects of the same type) (PR #77768)

[Yaxun Liu via cfe-commits]

yxsamliu wrote:

This PR caused a regression for AMDMIGraphX https://github.com/ROCm/AMDMIGraphX/blob/develop/src/targets/gpu/include/migraphx/gpu/prefix_scan_sum.hpp#L57

A reduced test case is:

#include <vector>
#include <stdio.h>
using namespace std;
```
struct A {
    int x;
    A(int x_) : x(x_) {}
    A(const vector<A>& a) {}
};

int main() {
    vector<A> a{1,2};
    vector<A> b{a};

    printf("%ld\n", b.size());
}
```
For variable b, it is expected to be constructed by copy ctor of vector, and have 2 elements.

With this patch, a is first converted to A(a), then b is constructed with one element A(a).

Is this expected or a bug? Thanks.

gcc has the same behavior https://godbolt.org/z/hTb795xWP

https://github.com/llvm/llvm-project/pull/77768