diff --git a/Makefile b/Makefile
index 297443a5..f16b209a 100644
--- a/Makefile
+++ b/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
diff --git a/include/nlohmann/detail/output/serializer.hpp b/include/nlohmann/detail/output/serializer.hpp
index 9994372d..74c2d771 100644
--- a/include/nlohmann/detail/output/serializer.hpp
+++ b/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);
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index ded6c2bc..8f61edc7 100644
--- a/single_include/nlohmann/json.hpp
+++ b/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);
diff --git a/test/src/unit-regression.cpp b/test/src/unit-regression.cpp
index 975b9239..822cac96 100644
--- a/test/src/unit-regression.cpp
+++ b/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]);
             }