Has there ever been any talk about making a concept extensible? i.e.
you can change it after its original definition?

For example, let's say that "std::arithmetic" were to start out as:

    template <typename T>
    concept arithmetic = is_arithmetic_v<T>;

but we mark it as 'extensible' to indicate that you're allowed to
change it later, something like:

    template <typename T>
    extensible concept arithmetic = is_arithmetic_v<T>;

Then someone comes along and writes one of those arbitrary precision
math libraries, and they have a class called "HugeInt", and they want
to tell the compiler that "HugeInt" is an arithmetic type, so in the
header file for "HugeInt", they write a line something like as
follows:

    concept arithmetic << HugeInt;

which changes the "std::arithmetic" concept as though it had
originally been written:

    template <typename T>
    concept arithmetic = is_arithmetic_v<T> || is_same_v<T, HugeInt>;

Of course, this would mean that "std::arithmetic" would have different
definitions in different translation units, but it wouldn't matter
because the translation units that don't include the HugeInt header
file won't ever use the HugeInt type.