Using version 2.6 of GSL from ftp://ftp.gnu.org/gnu/gsl/gsl-2.6.tar.gz

OS is Ubuntu 18.04.2 LTS

Hardware is a Dell workstation with a 3.60 GHz Intel i7-4790 and 32 GB
of RAM

Compiler is gcc 7.4.0 with the -w option

Description: when the first argument is NaN, gsl_sf_hypot appears to return the
product of its second argument with the square root of two which does
not seem to be expected behavior.

#include <gsl/gsl_sf.h>
#include <stdio.h>
#include <gsl/gsl_math.h>

int main (void){
double out;
out = gsl_sf_hypot(GSL_NAN, 1.0);

printf("%f\n", out); // prints out 1.41424
return 0;
}