Variable Length Array (VLA) in C++ compilers
The standard requires that a conforming compiler must "issue a diagnostic" when it encounters something that is illegal. Having done that, it's free to continue to compile the code with an implementation-specific meaning. (Note that "with an implementation-specific meaning" is a polite form of "with undefined behavior").
Why does the compiler accept that declaration?
Because its authors chose to make it do so.
GCC in particular allows, by default, a lot of non-standard stuff that was historically accepted by old C compilers. They like "compatibility" in that sense.
What does the standard say about [it]?
Precisely what the warning states it says about it: ISO C++ forbids variable length arrays.
C++ does not have VLAs.
Where you see one being accepted, it is a compiler extension; to find out how that compiler implements such an extension, you would have to ask the compiler's authors (or examine its source, if applicable).