🚨 fixed some warnings

Makefile

@@ -136,6 +136,7 @@ pedantic_clang:
 		-Wno-float-equal \
 		-Wno-switch-enum -Wno-covered-switch-default \
 		-Wno-c++2a-compat \
+		-Wno-c++17-extensions \
 		-Wno-padded"
 
 # calling GCC with most warnings

include/nlohmann/detail/output/serializer.hpp

@@ -620,7 +620,7 @@ class serializer
         if (is_negative)
         {
             *buffer_ptr = '-';
-            abs_value = static_cast<number_unsigned_t>(-1 - x) + 1;
+            abs_value = static_cast<number_unsigned_t>(std::abs(static_cast<std::intmax_t>(x)));
 
             // account one more byte for the minus sign
             n_chars = 1 + count_digits(abs_value);

single_include/nlohmann/json.hpp

@@ -11533,7 +11533,7 @@ class serializer
         if (is_negative)
         {
             *buffer_ptr = '-';
-            abs_value = static_cast<number_unsigned_t>(-1 - x) + 1;
+            abs_value = static_cast<number_unsigned_t>(std::abs(static_cast<std::intmax_t>(x)));
 
             // account one more byte for the minus sign
             n_chars = 1 + count_digits(abs_value);

test/src/unit-regression.cpp

@@ -1762,7 +1762,7 @@ TEST_CASE("regression tests")
                 -54,  -28,  -26
             };
             std::string s;
-            for (int i = 0; i < sizeof(data) / sizeof(int); i++)
+            for (unsigned i = 0; i < sizeof(data) / sizeof(int); i++)
             {
                 s += static_cast<char>(data[i]);
             }