[Bug c++/95656] New: Inconsistent output in compiling an undefined function

[haoxintu at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95656

            Bug ID: 95656
           Summary: Inconsistent output in compiling an undefined function
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: haoxintu at gmail dot com
  Target Milestone: ---

This code bug.cc

#include<iostream>
int main(){
    int a ();
    std::cout << a << std::endl;
    return 0;
}

In GCC-trunk without "-fsanitize=undefined"

$g++ bug.cc ; ./a.out
1

$g++-trunk -fsanitize=undefined bug.cc
/tmp/ccm15L4R.o: In function `main':
bug.cc:(.text+0x8f): undefined reference to `a()'
collect2: error: ld returned 1 exit status

While in GCC-5

$g++-5 bug.cc ; ./a.out
1
$g++-5 -fsanitize=undefined bug.cc ; ./a.out
1

My GCC versions are

$g++-trunk --version
g++ (GCC) 11.0.0 20200526 (experimental)
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$g++-5 --version
g++-5 (Ubuntu 5.5.0-12ubuntu6~18.04) 5.5.0 20171010
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

I also tested bug.cc in clang, it compiles well and output "1" whether adding
"-fsanitize=undefined" or not.