uninitialized string

[Manfred <mx2927 at gmail dot com>]

The following was asked on comp.lang.c++;
It compiles and crashes with no error/warning on gcc 8.2.1:

#include <iostream>
#include <string>

int main()
{
  std::string s1 { s1 };
  std::string s2(s2);
  std::string s3 = s3;

  std::cout << s1 << s2 << s3;
}

[tmp]$ g++ -Wall selfref.cc && ./a.out
Segmentation fault (core dumped)
[tmp]$

On the other hand:

#include <iostream>

int main()
{
  int n { n };

  std::cout  << n << std::endl;
}

[tmp]$ g++ -Wall selfint.cc && ./a.out
selfint.cc: In function ‘int main()’:
selfint.cc:5:7: warning: ‘n’ is used uninitialized in this function 
[-Wuninitialized]
   int n { n };
       ^
0
[tmp]$

I wonder why int triggers the warning, but std::string does not.