How to reproduce:

$ cat isnan.c 
#include <math.h>

int main(void)
{
  double foo = 1.0;

  if (isnan(foo))
    return 1;
  return 0;
}

$ clang -Wdouble-promotion -o isnan isnan.c
isnan.c:7:13: warning: implicit conversion increases floating-point precision: 'double' to 'long double' [-Wdouble-promotion]
  if (isnan(foo))
      ~~~~~~^~~~
/usr/include/math.h:644:46: note: expanded from macro 'isnan'
#  define isnan(x) __MATH_TG ((x), __isnan, (x))
                   ~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
/usr/include/math.h:562:16: note: expanded from macro '__MATH_TG'
   : FUNC ## l ARGS)
     ~~~~~~~~~ ^~~~
1 warning generated.



This is an issue with glibc 2.25 and newer.  I spent some time debugging this and it looks like in lib/Basic/DiagnosticIDs.cpp:471, the isInSystemHeader() function is returning false, because the SourceLocation points to isnan.c:7.  To make it more clear why there is a warning, here are the relevant parts of the source after running the preprocessor:


extern int __isnan (double __value) __attribute__ ((__nothrow__ )) __attribute__ ((__const__));
extern int __isnanf (float __value) __attribute__ ((__nothrow__ )) __attribute__ ((__const__));
extern int __isnanl (long double __value) __attribute__ ((__nothrow__ )) __attribute__ ((__const__));

int main(void)
{
  double foo = 1.0;

  if ((sizeof ((foo)) == sizeof (float) ? __isnanf (foo) : sizeof ((foo)) == sizeof (double) ? __isnan (foo) : __isnanl (foo)))
    return 1;
  return 0;
}