What's the meaning of 'int foo(int);'?

[watermelon123]

This question may sound strange. I'd found four kinds of initialization(value, default, direct, list) and tried many variable definitions which look similar. Below is one of my tries among them.

#include <iostream>

int main()
{
  int foo(int);
  std::cout << foo << std::endl;
}

Now I know 'int foo(int);' is not a definition of variable. It is a function declaration. But when I compiled it and run, terminal showed me the digit 1 without any diagnostic message. I didn't understand it because foo isn't defined anywhere. How is this output shown? Did I just invoke undefined behavior?

[seeplus]

Well, when I run it using VS2019 I get an expected linker error regarding unresolved symbol foo.

[jonnin]

the site compiler evaluates foo as 'true' with a warning. This seems like a minor bug.
if you call foo instead of printing its address, it is not happy.

[watermelon123]

Thank you seeplus and jonnin, I had to say I used g++ compiler.
I tried this code with VS2019 and the site compiler. I learned that the function pointer value is implicitly converted to bool value 'true' and VS2019 doesn't allow this code.

[kbw]

Clang's warning is:

kbw@squizey:/tmp % vim x.cc; sh -c 'CXXFGLAGS="-std=c++17 -g -Wall -Wextra" make x && ./x'
c++ -O2 -pipe  x.cc  -o x
x.cc:6:16: warning: address of function 'foo' will always evaluate to 'true' [-Wpointer-bool-conversion]
  std::cout << foo << std::endl;
            ~~ ^~~
x.cc:6:16: note: prefix with the address-of operator to silence this warning
  std::cout << foo << std::endl;
               ^
               &
1 warning generated.
1

g++

kbw@teppi:~/tmp2$ vim x.cc && CXXFLAGS="-std=c++17 -g -Wall -Wextra" make -B x && ./x
g++ -std=c++17 -g -Wall -Wextra    x.cc   -o x
x.cc: In function ‘int main()’:
x.cc:6:16: warning: the address of ‘int foo(int)’ will never be NULL [-Waddress]
    6 |   std::cout << foo << std::endl;
      |                ^~~
1

My expected behaviour is what VS is doing.

[dutch]

If you want to print the function address you need to cast it to void*.
This will definitely not work if the function is not defined.