$ g++ --version
Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/4.2.1
Apple clang version 12.0.0 (clang-1200.0.32.29)
Target: x86_64-apple-darwin23.4.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

a.cc

$ cat a.cc
#include<iostream>

using namespace std;

static int x = 5053;

void f2();

int main() {
  cout << "a: " << x << endl;
  f2();
  return 0;
}

b.cc

$ cat b.cc
#include<iostream>

using namespace std;

static int x = 4921;

string f2() {
  cout << "b: " << x << endl;
  return "";
}

./a.out

$ g++ --std=c++17 a.cc b.cc && ./a.out
a: 5053
b: 4921

Why was I able to forward declare string f2(); from b.cc as void f2(); in a.cc?
Any references to cppreference or spec that allows this would be appreciated.