#include<vector>
struct A{};
int main() {
    std::vector<A>a;
    a.emplace_back(0, 0);
}

Compiling this code with -std=c++17, the error below is shown:

/usr/include/c++/13/bits/new_allocator.h:187:11: error: new initializer expression list treated as compound expression [-fpermissive]
  187 |         { ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }
      |           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/13/bits/new_allocator.h:187:11: error: no matching function for call to ‘A::A(int)’
a.cpp:2:8: note: candidate: ‘constexpr A::A()’
    2 | struct A{};
      |        ^

However, after I provided A(int) for A, the compilers tried to call A::A(int, int):

/usr/include/c++/13/bits/new_allocator.h:187:11: error: no matching function for call to ‘A::A(int, int)’
  187 |         { ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }
      |           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cpp:3:5: note: candidate: ‘A::A(int)’
    3 |     A(int x) {}
      |     ^

Why did the compiler behave like that?