[Bug c++/113582] incorrect warning about unused label with `pragma GCC diagnostic` around the unused label

[mpolacek at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113582

--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
The reason I'm not doing simply

  TREE_USED (label) = TREE_USED (decl);

in the pt.cc/LABEL_EXPR hunk is that I think we still want the warning in the
second function, what with the goto being a discarded statement.

// PR c++/113582

template<bool B> void
do_something ()
{
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-label"
start:
  if constexpr(B)
    goto start;
#pragma GCC diagnostic pop
}

template<bool B> void
do_something2 ()
{
start: // { dg-warning "defined but not used" }
  if constexpr(B)
    goto start;
}

void
g ()
{
  do_something<0>();
  do_something2<0>();
}