sizeof taking two arguments

In C.1.3 of the C++ IS (2003. It's in the C++11 IS, too), the standard points out a difference between ISO C and C++; namely, for

char arr[100];

sizeof(0, arr) returns sizeof(char*) in C, but 100 in C++.

I can find no documentation for sizeof taking two arguments. The obvious fallback is the comma operator, but I don't think so: sizeof(arr) in C is 100; sizeof(0, arr) is sizeof(char*). Both sizeof(0, arr) and sizeof(arr) are 100 in C++.

I may be missing the whole point of the IS in this context. Can anyone help? This is similar to a question discussed back in '09, but no one referred to the IS, and I don't think the correct answer was given.

Edit: Actually, the IS is talking about the comma operator. So, for some reason (0, arr) returns a char* in C, but a char[100] in C++. Why?