The following snippet generates compilation errors on adding -pedantic and -Werror on compilers that are a bit old.

#include <cstdint>
#include <iostream>

int add(int a, int b){
    return a + b;
}; // <-- stray semicolon

int main (){
    return 0;
}

However this does not happen newer compiler versions. Please find a matrix of gcc (10.x, 11.x) and clang (5.x, 6.x) demonstrating the difference at https://godbolt.org/z/KWeb8WTxz.

I have two parts to my question:

1. Why is this not triggered in recent compilers?
2. Is it possible to enable the old behaviour in recent versions of clang or gcc?

>Solution :

Starting in C++11, extra semicolons ; (aka empty-declarations) at the global level are valid. I believe this is occasionally useful for writing macros.

As such, GCC 11 removed -pedantic diagnostics for an extra ; when -std=c++11 or later is used. See:

• [GCC Bug 96068] Extra semicolon outside of a function should be allowed after c++11?.
• [GCC Bugs Mailing list] -Wpedantic doesn’t warn about extra semicolons anymore
• [CWG 569] Spurious semicolons at namespace scope should be allowed

You can restore the old behavior by using a C++ standard older than C++11. Both GCC 11 and clang 6 will emit the old diagnostics if you pass -std=c++03.