Fix outputting extreme integer values in edge cases

For some gcc version (Ubuntu 5.5.0-12ubuntu1~16.04) the existing code
crashes when the minimum value of int64_t is outputted.

Resurrect the code from before 546e2cb (:rotating_light: fixed some warnings,
2019-03-13) but delegate the sign removal so that the compilers don't
complain about taking the negative value of an unsigned value.

In addition we also rewrite the expression so that we first increment
and then negate.

We on purpose don't supply a definition of
remove_sign(number_unsigned_t) as we only need the compiler to see the
declaration but never call the function.

include/nlohmann/detail/output/serializer.hpp

@@ -630,7 +630,7 @@ class serializer
         if (is_negative)
         {
             *buffer_ptr = '-';
-            abs_value = static_cast<number_unsigned_t>(std::abs(static_cast<std::intmax_t>(x)));
+            abs_value = remove_sign(x);
 
             // account one more byte for the minus sign
             n_chars = 1 + count_digits(abs_value);
@@ -811,6 +811,14 @@ class serializer
         return state;
     }
 
+    number_unsigned_t remove_sign(number_unsigned_t x);
+
+    inline number_unsigned_t remove_sign(number_integer_t x) noexcept
+    {
+        assert(x < 0 && x < (std::numeric_limits<number_integer_t>::max)());
+        return static_cast<number_unsigned_t>(-(x + 1)) + 1;
+    }
+
   private:
     /// the output of the serializer
     output_adapter_t<char> o = nullptr;

single_include/nlohmann/json.hpp

@@ -14264,7 +14264,7 @@ class serializer
         if (is_negative)
         {
             *buffer_ptr = '-';
-            abs_value = static_cast<number_unsigned_t>(std::abs(static_cast<std::intmax_t>(x)));
+            abs_value = remove_sign(x);
 
             // account one more byte for the minus sign
             n_chars = 1 + count_digits(abs_value);
@@ -14445,6 +14445,14 @@ class serializer
         return state;
     }
 
+    number_unsigned_t remove_sign(number_unsigned_t x);
+
+    inline number_unsigned_t remove_sign(number_integer_t x) noexcept
+    {
+        assert(x < 0 && x < (std::numeric_limits<number_integer_t>::max)());
+        return static_cast<number_unsigned_t>(-(x + 1)) + 1;
+    }
+
   private:
     /// the output of the serializer
     output_adapter_t<char> o = nullptr;