For the C++ code below, I am getting

error: specialization of 'static void ISR<N>::handler() [with unsigned int N = 5]' after instantiation
    9 | void ISR<5>::handler()
      |                   

when compiled with

$ avr-gcc -mmcu=atmega8 foo.cpp -mmcu=atmega8

Notice that signal(n) is a recent addition to GCC v15. I found Specialization of member function template after instantiation error, and order of member functions however I don’t see how to apply that solution to the code below.

template<unsigned N>
struct ISR
{
    __attribute__((__signal__(N)))
    static void handler();
};

template<>
void ISR<5>::handler()
{
    __asm ("nop");
}

int main (void)
{
  return 0;
}

The code compiles as expected without the function attribute.

What’s also strange is that it works with, say [[gnu::naked]] but not with [[gnu::signal]].