We are not checking yet if it's valid code. Just a first take

I think a TestSymbolDatabase test case would be useful to show that it works as intended. And then it would be interesting if you could run the test-my-pr.py script for a while and see what happens..

my_check_diff.log
256 packets were checked no diff

extern is not required to declare a function, see https://trac.cppcheck.net/ticket/11555
Do we handle e.g. extern int (i);?

extern is not required to declare a function, see https://trac.cppcheck.net/ticket/11555 Do we handle e.g. extern int (i);?

IsFunction checks for keywords. But overall the solution is for this ticket 12420 which is based around extern functions.

Do we handle e.g. extern int (i);?

Yes.. I tested this code:

void foo() {
    extern int x();  // <- cppcheck sets function pointer on "x"
    extern int (i);   // <- cppcheck does not set function pointer
}

extern is not required to declare a function, see https://trac.cppcheck.net/ticket/11555

I discussed that to introduce this more carefully.. but yes you are right..

This has caused a crash: https://trac.cppcheck.net/ticket/12432

Fixed with #5978