Consider the following example:
#include <format>
#include <print>
int main() {
int x = 12345;
// note: this is "awoo" followed by 28 zeros ([dcl.init.string] p3)
char buffer[32] = "awoo";
std::format_to(buffer, "{}{}", x, buffer);
std::println("{}", buffer);
}
This code is expected to output 12345awoo because we are formatting {}{} with arguments 12345 and "awoo".
However, the actual output is 1234512345. The reason for this is obvious: Formating 12345 would write to the buffer, and so by the time that buffer (the second argument) is being handled by std::format_to, it no longer contains "awoo".
This begs the question: What is the expected result according to the C++ standard? https://eel.is/c++draft/format.functions#lib:vformat_to does not specify as a precondition that aliasing between the arguments and the output buffer isn't permitted, so I don't believe that this case is undefined behavior.
It's unspecified, both 1234512345 and 12345awoo are conforming results.
The functions defined in [format.functions] use specializations of the class template formatter to format individual arguments.
Thus the formatting of each argument happens within a separate function call to the others. There isn't any requirement on the order of these calls, so the side-effects are indeterminately sequenced with each other.