/in/foo.cc: In function 'double abs(int)':
/in/foo.cc:7:15: error: ambiguating new declaration of 'double abs(int)'
 inline double abs (int x) {
               ^~~
In file included from /usr/include/c++/6/cstdlib:75:0,
                 from /usr/include/c++/6/ext/string_conversions.h:41,
                 from /usr/include/c++/6/bits/basic_string.h:5417,
                 from /usr/include/c++/6/string:52,
                 from /usr/include/c++/6/bits/locale_classes.h:40,
                 from /usr/include/c++/6/bits/ios_base.h:41,
                 from /usr/include/c++/6/ios:42,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/stdlib.h:735:12: note: old declaration 'int abs(int)'
 extern int abs (int __x) __THROW __attribute__ ((__const__)) __wur;
            ^~~