diff --git a/stl/inc/cstddef b/stl/inc/cstddef
index fb5fc321a14..561f916dd34 100644
--- a/stl/inc/cstddef
+++ b/stl/inc/cstddef
@@ -25,8 +25,9 @@ _STL_DISABLE_CLANG_WARNINGS
 #undef intrinsic
 
 _STD_BEGIN
-_EXPORT_STD using _CSTD ptrdiff_t;
-_EXPORT_STD using _CSTD size_t;
+// using deduction to define size_t and ptrdiff_t for better diagnostics, see GH-5699
+_EXPORT_STD using ptrdiff_t   = decltype((int*) (nullptr) - (int*) (nullptr));
+_EXPORT_STD using size_t      = decltype(sizeof(0));
 _EXPORT_STD using max_align_t = double; // most aligned type
 _EXPORT_STD using nullptr_t   = decltype(nullptr);
 
diff --git a/stl/inc/cstdio b/stl/inc/cstdio
index 674c36c3c75..c4a3b2188ec 100644
--- a/stl/inc/cstdio
+++ b/stl/inc/cstdio
@@ -32,7 +32,7 @@ _STD_BEGIN
 
 using _CSTD _Mbstatet;
 
-_EXPORT_STD using _CSTD size_t;
+_EXPORT_STD using size_t = decltype(sizeof(0));
 _EXPORT_STD using _CSTD fpos_t;
 _EXPORT_STD using _CSTD FILE;
 _EXPORT_STD using _CSTD clearerr;
diff --git a/stl/inc/cstdlib b/stl/inc/cstdlib
index 1bf0509fe3b..9e4f4a88d97 100644
--- a/stl/inc/cstdlib
+++ b/stl/inc/cstdlib
@@ -34,7 +34,7 @@ _NODISCARD _Check_return_ inline long double abs(_In_ long double _Xx) noexcept
 _END_EXTERN_CXX_WORKAROUND
 
 _STD_BEGIN
-_EXPORT_STD using _CSTD size_t;
+_EXPORT_STD using size_t = decltype(sizeof(0));
 _EXPORT_STD using _CSTD div_t;
 _EXPORT_STD using _CSTD ldiv_t;
 _EXPORT_STD using _CSTD abort;
diff --git a/stl/inc/cstring b/stl/inc/cstring
index 2445e9fbdfe..a6b264090d8 100644
--- a/stl/inc/cstring
+++ b/stl/inc/cstring
@@ -21,7 +21,7 @@ _STD_BEGIN
 #pragma warning(push)
 #pragma warning(disable : 4995) // name was marked as #pragma deprecated
 
-_EXPORT_STD using _CSTD size_t;
+_EXPORT_STD using size_t = decltype(sizeof(0));
 _EXPORT_STD using _CSTD memchr;
 _EXPORT_STD using _CSTD memcmp;
 _EXPORT_STD using _CSTD memcpy;
diff --git a/stl/inc/ctime b/stl/inc/ctime
index ae2f152edc4..3611460d0bf 100644
--- a/stl/inc/ctime
+++ b/stl/inc/ctime
@@ -19,7 +19,7 @@ _STL_DISABLE_CLANG_WARNINGS
 
 _STD_BEGIN
 _EXPORT_STD using _CSTD clock_t;
-_EXPORT_STD using _CSTD size_t;
+_EXPORT_STD using size_t = decltype(sizeof(0));
 _EXPORT_STD using _CSTD time_t;
 _EXPORT_STD using _CSTD tm;
 _EXPORT_STD using _CSTD asctime;
diff --git a/stl/inc/cuchar b/stl/inc/cuchar
index c7820977427..f75dff0f199 100644
--- a/stl/inc/cuchar
+++ b/stl/inc/cuchar
@@ -19,7 +19,7 @@ _STL_DISABLE_CLANG_WARNINGS
 
 _STD_BEGIN
 _EXPORT_STD using _CSTD mbstate_t;
-_EXPORT_STD using _CSTD size_t;
+_EXPORT_STD using size_t = decltype(sizeof(0));
 _EXPORT_STD using _CSTD mbrtoc16;
 _EXPORT_STD using _CSTD c16rtomb;
 _EXPORT_STD using _CSTD mbrtoc32;
diff --git a/stl/inc/cwchar b/stl/inc/cwchar
index 5b2574db37b..c80a6ab16d9 100644
--- a/stl/inc/cwchar
+++ b/stl/inc/cwchar
@@ -29,7 +29,7 @@ _STD_BEGIN
 using _CSTD _Mbstatet;
 
 _EXPORT_STD using _CSTD mbstate_t;
-_EXPORT_STD using _CSTD size_t;
+_EXPORT_STD using size_t = decltype(sizeof(0));
 _EXPORT_STD using _CSTD tm;
 _EXPORT_STD using _CSTD wint_t;