It is a bugfix proposal for https://bugs.llvm.org/show_bug.cgi?id=42166.
std::to_chars appends leading zeros if input 64-bit value has 9, 10 or 11 digits.
According to documentation std::to_chars must not append leading zeros:
Changeset should not affect std::to_chars performance:
Unit test that std::from_chars supports compatibility for both std::to_chars outputs (previous and fixed one) already exists: