foo.cc: In function 'I64 abs(I64)':
foo.cc:12:14: error: 'I64 abs(I64)' conflicts with a previous declaration
 I64 abs(I64 x)
              ^
In file included from /usr/include/c++/7/cstdlib:77:0,
                 from /usr/include/c++/7/ext/string_conversions.h:41,
                 from /usr/include/c++/7/bits/basic_string.h:6361,
                 from /usr/include/c++/7/string:52,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/std_abs.h:61:3: note: previous declaration 'long long int std::abs(long long int)'
   abs(long long __x) { return __builtin_llabs (__x); }
   ^~~
foo.cc: In function 'I64 myrand()':
foo.cc:25:13: error: call of overloaded 'abs(I64&)' is ambiguous
 return abs(a);
             ^
In file included from /usr/include/c++/7/cstdlib:75:0,
                 from /usr/include/c++/7/ext/string_conversions.h:41,
                 from /usr/include/c++/7/bits/basic_string.h:6361,
                 from /usr/include/c++/7/string:52,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/stdlib.h:837:12: note: candidate: int abs(int)
 extern int abs (int __x) __THROW __attribute__ ((__const__)) __wur;
            ^~~
In file included from /usr/include/c++/7/cstdlib:77:0,
                 from /usr/include/c++/7/ext/string_conversions.h:41,
                 from /usr/include/c++/7/bits/basic_string.h:6361,
                 from /usr/include/c++/7/string:52,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/std_abs.h:56:3: note: candidate: long int std::abs(long int)
   abs(long __i) { return __builtin_labs(__i); }
   ^~~
/usr/include/c++/7/bits/std_abs.h:61:3: note: candidate: long long int std::abs(long long int)
   abs(long long __x) { return __builtin_llabs (__x); }
   ^~~
/usr/include/c++/7/bits/std_abs.h:70:3: note: candidate: constexpr double std::abs(double)
   abs(double __x)
   ^~~
/usr/include/c++/7/bits/std_abs.h:74:3: note: candidate: constexpr float std::abs(float)
   abs(float __x)
   ^~~
/usr/include/c++/7/bits/std_abs.h:78:3: note: candidate: constexpr long double std::abs(long double)
   abs(long double __x)
   ^~~
foo.cc:12:5: note: candidate: I64 abs(I64)
 I64 abs(I64 x)
     ^~~
foo.cc: In function 'I64 gcd(I64, I64)':
foo.cc:44:20: error: call of overloaded 'abs(I64&)' is ambiguous
 if(!y) return abs(x);
                    ^
In file included from /usr/include/c++/7/cstdlib:75:0,
                 from /usr/include/c++/7/ext/string_conversions.h:41,
                 from /usr/include/c++/7/bits/basic_string.h:6361,
                 from /usr/include/c++/7/string:52,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/stdlib.h:837:12: note: candidate: int abs(int)
 extern int abs (int __x) __THROW __attribute__ ((__const__)) __wur;
            ^~~
In file included from /usr/include/c++/7/cstdlib:77:0,
                 from /usr/include/c++/7/ext/string_conversions.h:41,
                 from /usr/include/c++/7/bits/basic_string.h:6361,
                 from /usr/include/c++/7/string:52,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/c++/7/bits/std_abs.h:56:3: note: candidate: long int std::abs(long int)
   abs(long __i) { return __builtin_labs(__i); }
   ^~~
/usr/include/c++/7/bits/std_abs.h:61:3: note: candidate: long long int std::abs(long long int)
   abs(long long __x) { return __builtin_llabs (__x); }
   ^~~
/usr/include/c++/7/bits/std_abs.h:70:3: note: candidate: constexpr double std::abs(double)
   abs(double __x)
   ^~~
/usr/include/c++/7/bits/std_abs.h:74:3: note: candidate: constexpr float std::abs(float)
   abs(float __x)
   ^~~
/usr/include/c++/7/bits/std_abs.h:78:3: note: candidate: constexpr long double std::abs(long double)
   abs(long double __x)
   ^~~
foo.cc:12:5: note: candidate: I64 abs(I64)
 I64 abs(I64 x)
     ^~~
foo.cc: In function 'void output()':
foo.cc:114:24: warning: format '%d' expects argument of type 'int', but argument 2 has type 'I64 {aka long long int}' [-Wformat=]
 printf("%I64d\n",ans[0]);
                  ~~~~~~^
foo.cc: In function 'I64 Pollard(I64)':
foo.cc:94:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。