[Bug c++/94175] New: [10 Regression] Passing constexpr empty class variable to function since r10-599

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94175

            Bug ID: 94175
           Summary: [10 Regression] Passing constexpr empty class variable
                    to function since r10-599
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jakub at gcc dot gnu.org
  Target Milestone: ---

Since r10-599-gc652ff83124334837dc16626f9e1040e4fe41fc9 following testcase with
-O2 -m32 (or -O0 -m32 or any arch other than x86_64 64-bit):
struct Foo
{
  struct NoStartBar {};
  static constexpr NoStartBar noStartBar = NoStartBar();
  [[gnu::noinline, gnu::noclone]] Foo(int x, NoStartBar) : f (x) { f++; }
  Foo(double x) : Foo(0, noStartBar) {}
  int f;
};
Foo a = 6.0;
needs the Foo::noStartBar definition, while before it didn't and just passed
NoStartBar {}.
Is passing the static data member an ODR use of it or not?
In any case, looks like a missed optimization, there shouldn't be reason to
copy the empty class from a variable rather than just passing uninitialized
bytes.