German Wikipedia states that it's disputed, if the cube root of a negative real number was defined
I finally got around to taking a look at the C89, C99 and C11 standards.
There's no mention of "cbrt" in C89, but in C99 and C11 it's quite acceptable for cbrt/cbrtf/cbrtl to take a negative argument.
However, if pow/powf/powl are given a first argument that is negative && the second argument has a non-integer value, then a "domain error" occurs.