diff --git a/libcxx/include/charconv b/libcxx/include/charconv
--- a/libcxx/include/charconv
+++ b/libcxx/include/charconv
@@ -93,8 +93,8 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 namespace __itoa {
-_LIBCPP_AVAILABILITY_TO_CHARS _LIBCPP_FUNC_VIS char* __u64toa(uint64_t __value, char* __buffer);
-_LIBCPP_AVAILABILITY_TO_CHARS _LIBCPP_FUNC_VIS char* __u32toa(uint32_t __value, char* __buffer);
+_LIBCPP_AVAILABILITY_TO_CHARS _LIBCPP_FUNC_VIS char* __u64toa(uint64_t __value, char* __buffer) _NOEXCEPT;
+_LIBCPP_AVAILABILITY_TO_CHARS _LIBCPP_FUNC_VIS char* __u32toa(uint32_t __value, char* __buffer) _NOEXCEPT;
 }
 
 #ifndef _LIBCPP_CXX03_LANG
diff --git a/libcxx/src/charconv.cpp b/libcxx/src/charconv.cpp
--- a/libcxx/src/charconv.cpp
+++ b/libcxx/src/charconv.cpp
@@ -99,7 +99,7 @@
 }
 
 char*
-__u32toa(uint32_t value, char* buffer)
+__u32toa(uint32_t value, char* buffer) _NOEXCEPT
 {
     if (value < 100000000)
     {
@@ -120,7 +120,7 @@
 }
 
 char*
-__u64toa(uint64_t value, char* buffer)
+__u64toa(uint64_t value, char* buffer) _NOEXCEPT
 {
     if (value < 100000000)
     {