Improves both the compile-time and run-time errors.
At compile-time it does a bit more work to get more specific errors.
This could be done at run-time too, but that has a performance penalty.
Since it's expected most use-cases use format* instead of vformat* the
compile-time errors are more common.
For example when using
std::format_to("{:-c}", 42);
Before compile output would contain
std::__throw_format_error("The format-spec should consume the input or end with a '}'");
Now it contains
std::__throw_format_error("The format specifier does not allow the sign option");
Given a better indication the sign option is not allowed. Note the
output is still not user-friendly; C++ doesn't have good facilities to
generate nice messages from the library.
In general all messages have been reviewed and improved, using a more
consistent style and using less terms used in the standard. For example
format-spec -> format specifier arg-id -> argument index
Elsewhere too.