Implement opUDAOn and opChildOfUDAOn

[rikkimax]

Is part of linker list work #22824.

Requires DIP.

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[nordlow]

What are the use case(s) for this other than

struct UDA {
    void opUDAOn(alias symbol)() {
        pragma(msg, "On: " ~ __traits(identifier, symbol));
    }

    void opChildOfUDAOn(alias symbol)() {
        pragma(msg, "Child: " ~ __traits(identifier, symbol));
    }
}

?

[rikkimax]

What are the use case(s) for this other than

struct UDA {
    void opUDAOn(alias symbol)() {
        pragma(msg, "On: " ~ __traits(identifier, symbol));
    }

    void opChildOfUDAOn(alias symbol)() {
        pragma(msg, "Child: " ~ __traits(identifier, symbol));
    }
}

?

This doesn't seem like a complete question.

The purpose of these two operator overloads is to allow triggering of reflection capabilities that are then registered somewhere.

Such as with web routing or deserialization.