struct S { int a; };
void baz (S *);
#if __cpp_constexpr >= 200704
constexpr
#endif
inline S foo () { return (S) { 2 }; }

S s = foo ();

for -std=c++98 -O2 we have the runtime initializer.  If we had some pass that would analyze the ctor functions after they were inlined into and if they only write constants into file-scope vars defined in the current TU (except for comdat vars?) turn the ctor function into static initializers for the file-scope vars.
Not sure if we can do it generally for any (non-comdat?) vars, or if e.g. the C++ FE wouldn't have to mark such vars some way for us, or if we wouldn't need to analyze if there aren't some ctors in the same TU that would run earlier (higher (or is that lower?) priority) and could access those vars, or for vars visible outside of the TU if some other TU's ctor couldn't access them.
An argument for some guidance from the C++ FE would be that say in C:
int a __attribute__((nocommon));
static __attribute__((constructor)) void
foo (void)
{
  a = 6;
}
some other TU could say in heither priority ctor expect to see a == 0.