[Bug c++/81429] maybe_unused attribute triggers syntax error when used on first argument to a constructor

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81429

--- Comment #6 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
I have a fix.  Extended testcase:

void fn1([[maybe_unused]] int a) { }
void fn2(int a [[maybe_unused]]) { }
void fn3(__attribute__((unused)) int a) { }
void fn4(int a __attribute__((unused))) { }

struct S1 {
  S1([[maybe_unused]] int a) { }
};

struct S2 {
  S2(int f,  [[maybe_unused]] int a) { }
};

struct S3 {
  S3(int a [[maybe_unused]]) { }
};

struct S4 {
  S4(int f, int a [[maybe_unused]]) { }
};

struct S5 {
  S5(__attribute__((unused)) int a) { }
};

struct S6 {
  S6(int f, __attribute__((unused)) int a) { }
};

struct S7 {
  S7(int a __attribute__((unused))) { }
};

struct S8 {
  S8(int f, int a __attribute__((unused))) { }
};