[format.string.std] doesn't specify if implementations should consider the estimated width of the fill character when substituting it into the formatted results.

For example:

auto s = std::format("{:🤡>10}", 42);

"🤡" (U+1F921) is a single code point but its estimated display width is two.

There are at least three possible resolutions:

1. s == "🤡🤡🤡🤡42": use the estimated display width, correctly displayed on compatible terminals.

2. s == "🤡🤡🤡🤡🤡🤡🤡🤡42": assume the display width of 1, incorrectly displayed.

3. Require the fill character to have the estimated width of 1.