LWG3639: Handling of fill character width is underspecified in std::format #1150
Description
SG16 needs to clarify the intended design for https://cplusplus.github.io/LWG/issue3639.
20.20.2.2 [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.