Consider the following example:

#include <format>
#include <print>

int main() {
    int x = 12345;
    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.