format-family methods should accept args as const references

[0x804d8000]

#2462 fixed an inconsistency between signatures of format-family methods, in a way that now all of them accept args as universal references.

Unfortunately, universal references are not quite "universal" in that they can't be bound to bit fields, e.g.:

struct C {
  int f : 1;
};

C c = {.f = 1};

void foo() {
  char buf[10];
  format_to_n(buf, 10, "{}", c.f);  // Compiles before #2462, no longer compiles now.
}

This might be a long-standing issue with format and format_to, I suppose.

To fix this, I suggest that we should instead accept args as const references for all of format-family methods.

[ethanvc]

人家才统一改成 universal referenece，你就来拆台了。

[foonathan]

The choice to use forwarding references here is intentional to support formatting of ranges, see: https://wg21.link/p2418

[0x804d8000]

Thanks for mentioning P2418, I wasn't aware of it.