[Bug c++/103912] ICE in a consteval function which returns a lambda which takes a "non-POD" argument and the consteval has other code

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103912

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
This happens because of:
              /* Preserve a functions function context node.  It will
                 later be needed to output debug info.  */
              if (tree fn = decl_function_context (decl))
                {
                  cgraph_node *origin_node = cgraph_node::get_create (fn);
                  enqueue_node (origin_node);
                }
in cgraphunit.c, operator() of the lambda is needed, so it is enqueued and the
above forces gimplification of the consteval function too, which is a big no
no, such functions shouldn't exist.

The big question is what do we want to do for debug info in these cases.

When using instead:
struct A {A (); };

constexpr auto
foo ()
{
  if (1)
    ;
  return [] (A x) { };
}

void
bar (A x)
{
  constinit static auto h = foo ();
  h (x);
}
ipa.c discovers that foo isn't reachable even when it is needed (as function
context of the lambda) and throws away the body and in the debug info just
emits DW_TAG_subprogram etc. for foo, but only emits non-code related stuff for
it.