I wanted to bring this new LWG issue to your attention.
The author asks whether the fill character of std::format is
- a code unit
- a code point
- a grapheme cluster
This might be an abi breaking thing, and implementation disagrees already apparently.
My gut feeling is that it needs to at least be a codepoint.
I do not know if there are any concerns with allowing a grapheme in terms of implementation or performance. There is definitively some motivation, especially for non-nfc format strings.
This sort of issue illustrates my point that using the term character in the standard can be problematic!
Have a great week,