What blocks implementation of std::to_chars and std::from_chars

The to/from_chars feature requires that implementations provide round-tripping guarantees (with themselves). Specifically, the following must work:

float f = //get some float
char chars[LOTS_OF_CHARS];
auto result = to_chars(chars, chars + sizeof(chars), f);
float g;
from_chars(chars, result.ptr, g);
assert(f == g);

That guarantee is actually kind of hard to implement, and none of the standard library C or C++ float-to-string-to-float functions have ever provided that guarantee. So you can't just take code from printf/scanf or stof/to_string, rip out the locale stuff, and call that a to/from_chars implementation.