Why [[likely]] attribute in C++20 raises a warning here?

#include <iostream>

int foo(int a, int b)
{
    if(a < b) [[likely]] {
        return a;
    }
    return b;
}

int main()
{
    std::cout << foo(3,1) << std::endl;
}

Demo

According to the reference, it seems like this is how we're supposed to decorate if clauses with [[likely]] or [[unlikely]] attributes. It's also supported in C++20 (see here).

However, I'm running into a warning:

main.cpp: In function 'int foo(int, int)':
main.cpp:5:15: warning: attributes at the beginning of statement are ignored [-Wattributes]
    5 |     if(a < b) [[likely]] {
      |               ^~~~~~~~~~

The codebase is strict about warnings, and this will cause builds to fail. So, am I doing something wrong, or is this a bug?

g++ version on my macbook:

g++-9 (Homebrew GCC 9.3.0_1) 9.3.0
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