Comment (by Roland Haas):

It turns out it wants a cast to vector double * (see http://openpowerfoundation.org/wp-content/uploads/resources/Intrinsics-Reference/Intrinsics-Reference-20200211.pdf which does list both vec_st(vector double x, long long int, double * ) and vec_st(vector double x, long long int, vector double * ) and apparently gcc 8 only supports the latter one). So this:

#if 0
g++ -c -g3 -mcpu=power9 -mvsx -O1 -Wall $@ $0
exit
#endif

#include <altivec.h>

void foo(vector double x, double &p, double &q, double &r, double &s)
{
  const long long null = 0;
  *(vector double*)&p = x;
  
--

Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2530/thorn-vectors-fails-to-compile-using-gcc