Pretty straightforward.  With -maltivec -Wunused-but-set-parameter, this works in C but emits a diagnostic in C++:

  #include <altivec.h>
  int f(vector int b) {
    return vec_extract(b, 0);
  }

FWIW, the same problem happens if b is a local variable not a parameter, though of course it will emit an unused-but-set-variable diagnostic instead.