value not set via reference

[Conrad S <conradsand dot arma at gmail dot com>]

Which compiler is correct here - gcc or clang?
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64870

Consider the following code:

#include <iostream>

struct blah {
  inline double setval(unsigned int& x) const
    {
    x = 123;
    return 456.0;
    }
  };


int
main(int argc, char** argv) {
  blah blah_instance;

  unsigned int val = 9999;

  std::cout << blah_instance.setval(val) << "  val: " << val << std::endl;
  std::cout << blah_instance.setval(val) << "  val: " << val << std::endl;

  return 0;
  }

when compiled with gcc 4.9.2, the above program produces:
456  val: 9999  <-- unexpected
456  val: 123

when compiled with clang 3.5:
456  val: 123
456  val: 123

Clang has the least surprising result. Is gcc relying on a loophole in
C++ legalese to muck up the order of evaluation?