Comment # 1 on bug 1056783 from Richard Biener

I suppose that the recent glibc update might have broken this.  G++ 6 libstdc++
has the following which means that eventually G++ 6 needs to be re-built?

#ifndef __CORRECT_ISO_CPP11_MATH_H_PROTO_FP
  constexpr bool
  isnan(float __x)
  { return __builtin_isnan(__x); }

#if _GLIBCXX_HAVE_OBSOLETE_ISNAN \
  && !_GLIBCXX_NO_OBSOLETE_ISINF_ISNAN_DYNAMIC
  using ::isnan;
#else
  constexpr bool
  isnan(double __x)
  { return __builtin_isnan(__x); }
#endif

  constexpr bool
  isnan(long double __x)
  { return __builtin_isnan(__x); }
#endif