Why is a stray semicolon no longer detected by `-pedantic` modern compilers?

Question

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:

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

Because it's legal in all declaration contexts now. It wasn't before C++14 (in class scope) even though compiles allowed it. — 1 hour ago

Jan Schultke 21.6k7 gold badges53 silver badges109 bronze badges · Accepted Answer · 2023-10-08 17:39:41Z

7

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:

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.

edited 11 hours ago

Jan Schultke

21.6k7 gold badges53 silver badges109 bronze badges

answered 11 hours ago

Brian61354270

9,4125 gold badges22 silver badges44 bronze badges

2

Thank you. My code uses cpp20, so -std=c++03 is not viable. I am getting this on a CentOS compiler where I build python wheels. Any other static-analysis available that you know of that can help? Could you provide an example where this is useful for macros, as mentioned in this answer?

– jerin

10 hours ago
3

@jerin you could have something like an IF_DEBUG macro which removes code outside debug builds. Then, IF_DEBUG(int counter = 0); to declare a debug-only global variable. You could of course put the semicolon inside the macro argument list, but this may mess up auto-formatting with clang-format, and having the semicolon after a function-style macro is preferred by most developers. On a non-debug build, this macro would expand to nothing and you would have an empty-declaration.

– Jan Schultke

10 hours ago

Why is a stray semicolon no longer detected by `-pedantic` modern compilers?