diff --git a/stl/inc/xstring b/stl/inc/xstring
index 8ba06e0de21..46dfaeaca59 100644
--- a/stl/inc/xstring
+++ b/stl/inc/xstring
@@ -2512,20 +2512,31 @@ public:
         }
     }
 
+private:
+    _NODISCARD static _CONSTEXPR20 size_type _Find_result_cast(const size_t _Off) noexcept {
+        if constexpr (sizeof(size_type) > sizeof(size_t)) { // only happens on 32-bit platforms with 64-bit size_type
+            if (_Off == static_cast<size_t>(-1)) {
+                return npos;
+            }
+        }
+        return static_cast<size_type>(_Off);
+    }
+
+public:
 #if _HAS_CXX17
     template <class _StringViewIsh, _Is_string_view_ish<_StringViewIsh> = 0>
     _NODISCARD _CONSTEXPR20 size_type find(const _StringViewIsh& _Right, const size_type _Off = 0) const
         noexcept(_Is_nothrow_convertible_v<const _StringViewIsh&, basic_string_view<_Elem, _Traits>>) {
         // look for _Right beginning at or after _Off
         basic_string_view<_Elem, _Traits> _As_view = _Right;
-        return static_cast<size_type>(_Traits_find<_Traits>(_Mypair._Myval2._Myptr(),
+        return _Find_result_cast(_Traits_find<_Traits>(_Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _As_view.data(), _As_view.size()));
     }
 #endif // _HAS_CXX17
 
     _NODISCARD _CONSTEXPR20 size_type find(const basic_string& _Right, const size_type _Off = 0) const noexcept {
         // look for _Right beginning at or after _Off
-        return static_cast<size_type>(_Traits_find<_Traits>(_Mypair._Myval2._Myptr(),
+        return _Find_result_cast(_Traits_find<_Traits>(_Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _Right._Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Right._Mypair._Myval2._Mysize)));
     }
@@ -2533,7 +2544,7 @@ public:
     _NODISCARD _CONSTEXPR20 size_type find(_In_reads_(_Count) const _Elem* const _Ptr, const size_type _Off,
         const size_type _Count) const noexcept /* strengthened */ {
         // look for [_Ptr, _Ptr + _Count) beginning at or after _Off
-        return static_cast<size_type>(
+        return _Find_result_cast(
             _Traits_find<_Traits>(_Mypair._Myval2._Myptr(), static_cast<size_t>(_Mypair._Myval2._Mysize),
                 static_cast<size_t>(_Off), _Ptr, static_cast<size_t>(_Count)));
     }
@@ -2541,13 +2552,13 @@ public:
     _NODISCARD _CONSTEXPR20 size_type find(_In_z_ const _Elem* const _Ptr, const size_type _Off = 0) const noexcept
     /* strengthened */ {
         // look for [_Ptr, <null>) beginning at or after _Off
-        return static_cast<size_type>(_Traits_find<_Traits>(_Mypair._Myval2._Myptr(),
+        return _Find_result_cast(_Traits_find<_Traits>(_Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _Ptr, _Traits::length(_Ptr)));
     }
 
     _NODISCARD _CONSTEXPR20 size_type find(const _Elem _Ch, const size_type _Off = 0) const noexcept {
         // look for _Ch at or after _Off
-        return static_cast<size_type>(_Traits_find_ch<_Traits>(
+        return _Find_result_cast(_Traits_find_ch<_Traits>(
             _Mypair._Myval2._Myptr(), static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _Ch));
     }
 
@@ -2557,14 +2568,14 @@ public:
         noexcept(_Is_nothrow_convertible_v<const _StringViewIsh&, basic_string_view<_Elem, _Traits>>) {
         // look for _Right beginning before _Off
         basic_string_view<_Elem, _Traits> _As_view = _Right;
-        return static_cast<size_type>(_Traits_rfind<_Traits>(_Mypair._Myval2._Myptr(),
+        return _Find_result_cast(_Traits_rfind<_Traits>(_Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _As_view.data(), _As_view.size()));
     }
 #endif // _HAS_CXX17
 
     _NODISCARD _CONSTEXPR20 size_type rfind(const basic_string& _Right, const size_type _Off = npos) const noexcept {
         // look for _Right beginning before _Off
-        return static_cast<size_type>(_Traits_rfind<_Traits>(_Mypair._Myval2._Myptr(),
+        return _Find_result_cast(_Traits_rfind<_Traits>(_Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _Right._Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Right._Mypair._Myval2._Mysize)));
     }
@@ -2572,7 +2583,7 @@ public:
     _NODISCARD _CONSTEXPR20 size_type rfind(_In_reads_(_Count) const _Elem* const _Ptr, const size_type _Off,
         const size_type _Count) const noexcept /* strengthened */ {
         // look for [_Ptr, _Ptr + _Count) beginning before _Off
-        return static_cast<size_type>(
+        return _Find_result_cast(
             _Traits_rfind<_Traits>(_Mypair._Myval2._Myptr(), static_cast<size_t>(_Mypair._Myval2._Mysize),
                 static_cast<size_t>(_Off), _Ptr, static_cast<size_t>(_Count)));
     }
@@ -2580,13 +2591,13 @@ public:
     _NODISCARD _CONSTEXPR20 size_type rfind(_In_z_ const _Elem* const _Ptr, const size_type _Off = npos) const noexcept
     /* strengthened */ {
         // look for [_Ptr, <null>) beginning before _Off
-        return static_cast<size_type>(_Traits_rfind<_Traits>(_Mypair._Myval2._Myptr(),
+        return _Find_result_cast(_Traits_rfind<_Traits>(_Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _Ptr, _Traits::length(_Ptr)));
     }
 
     _NODISCARD _CONSTEXPR20 size_type rfind(const _Elem _Ch, const size_type _Off = npos) const noexcept {
         // look for _Ch before _Off
-        return static_cast<size_type>(_Traits_rfind_ch<_Traits>(
+        return _Find_result_cast(_Traits_rfind_ch<_Traits>(
             _Mypair._Myval2._Myptr(), static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _Ch));
     }
 
@@ -2596,7 +2607,7 @@ public:
         noexcept(_Is_nothrow_convertible_v<const _StringViewIsh&, basic_string_view<_Elem, _Traits>>) {
         // look for one of _Right at or after _Off
         basic_string_view<_Elem, _Traits> _As_view = _Right;
-        return static_cast<size_type>(_Traits_find_first_of<_Traits>(_Mypair._Myval2._Myptr(),
+        return _Find_result_cast(_Traits_find_first_of<_Traits>(_Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _As_view.data(), _As_view.size()));
     }
 #endif // _HAS_CXX17
@@ -2604,7 +2615,7 @@ public:
     _NODISCARD _CONSTEXPR20 size_type find_first_of(
         const basic_string& _Right, const size_type _Off = 0) const noexcept {
         // look for one of _Right at or after _Off
-        return static_cast<size_type>(_Traits_find_first_of<_Traits>(_Mypair._Myval2._Myptr(),
+        return _Find_result_cast(_Traits_find_first_of<_Traits>(_Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _Right._Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Right._Mypair._Myval2._Mysize)));
     }
@@ -2612,7 +2623,7 @@ public:
     _NODISCARD _CONSTEXPR20 size_type find_first_of(_In_reads_(_Count) const _Elem* const _Ptr, const size_type _Off,
         const size_type _Count) const noexcept /* strengthened */ {
         // look for one of [_Ptr, _Ptr + _Count) at or after _Off
-        return static_cast<size_type>(
+        return _Find_result_cast(
             _Traits_find_first_of<_Traits>(_Mypair._Myval2._Myptr(), static_cast<size_t>(_Mypair._Myval2._Mysize),
                 static_cast<size_t>(_Off), _Ptr, static_cast<size_t>(_Count)));
     }
@@ -2620,13 +2631,13 @@ public:
     _NODISCARD _CONSTEXPR20 size_type find_first_of(
         _In_z_ const _Elem* const _Ptr, const size_type _Off = 0) const noexcept /* strengthened */ {
         // look for one of [_Ptr, <null>) at or after _Off
-        return static_cast<size_type>(_Traits_find_first_of<_Traits>(_Mypair._Myval2._Myptr(),
+        return _Find_result_cast(_Traits_find_first_of<_Traits>(_Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _Ptr, _Traits::length(_Ptr)));
     }
 
     _NODISCARD _CONSTEXPR20 size_type find_first_of(const _Elem _Ch, const size_type _Off = 0) const noexcept {
         // look for _Ch at or after _Off
-        return static_cast<size_type>(_Traits_find_ch<_Traits>(
+        return _Find_result_cast(_Traits_find_ch<_Traits>(
             _Mypair._Myval2._Myptr(), static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _Ch));
     }
 
@@ -2636,14 +2647,14 @@ public:
         noexcept(_Is_nothrow_convertible_v<const _StringViewIsh&, basic_string_view<_Elem, _Traits>>) {
         // look for one of _Right before _Off
         basic_string_view<_Elem, _Traits> _As_view = _Right;
-        return static_cast<size_type>(_Traits_find_last_of<_Traits>(_Mypair._Myval2._Myptr(),
+        return _Find_result_cast(_Traits_find_last_of<_Traits>(_Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _As_view.data(), _As_view.size()));
     }
 #endif // _HAS_CXX17
 
     _NODISCARD _CONSTEXPR20 size_type find_last_of(const basic_string& _Right, size_type _Off = npos) const noexcept {
         // look for one of _Right before _Off
-        return static_cast<size_type>(_Traits_find_last_of<_Traits>(_Mypair._Myval2._Myptr(),
+        return _Find_result_cast(_Traits_find_last_of<_Traits>(_Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _Right._Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Right._Mypair._Myval2._Mysize)));
     }
@@ -2651,7 +2662,7 @@ public:
     _NODISCARD _CONSTEXPR20 size_type find_last_of(_In_reads_(_Count) const _Elem* const _Ptr, const size_type _Off,
         const size_type _Count) const noexcept /* strengthened */ {
         // look for one of [_Ptr, _Ptr + _Count) before _Off
-        return static_cast<size_type>(
+        return _Find_result_cast(
             _Traits_find_last_of<_Traits>(_Mypair._Myval2._Myptr(), static_cast<size_t>(_Mypair._Myval2._Mysize),
                 static_cast<size_t>(_Off), _Ptr, static_cast<size_t>(_Count)));
     }
@@ -2659,13 +2670,13 @@ public:
     _NODISCARD _CONSTEXPR20 size_type find_last_of(
         _In_z_ const _Elem* const _Ptr, const size_type _Off = npos) const noexcept /* strengthened */ {
         // look for one of [_Ptr, <null>) before _Off
-        return static_cast<size_type>(_Traits_find_last_of<_Traits>(_Mypair._Myval2._Myptr(),
+        return _Find_result_cast(_Traits_find_last_of<_Traits>(_Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _Ptr, _Traits::length(_Ptr)));
     }
 
     _NODISCARD _CONSTEXPR20 size_type find_last_of(const _Elem _Ch, const size_type _Off = npos) const noexcept {
         // look for _Ch before _Off
-        return static_cast<size_type>(_Traits_rfind_ch<_Traits>(
+        return _Find_result_cast(_Traits_rfind_ch<_Traits>(
             _Mypair._Myval2._Myptr(), static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _Ch));
     }
 
@@ -2675,7 +2686,7 @@ public:
         noexcept(_Is_nothrow_convertible_v<const _StringViewIsh&, basic_string_view<_Elem, _Traits>>) {
         // look for none of _Right at or after _Off
         basic_string_view<_Elem, _Traits> _As_view = _Right;
-        return static_cast<size_type>(_Traits_find_first_not_of<_Traits>(_Mypair._Myval2._Myptr(),
+        return _Find_result_cast(_Traits_find_first_not_of<_Traits>(_Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _As_view.data(), _As_view.size()));
     }
 #endif // _HAS_CXX17
@@ -2683,7 +2694,7 @@ public:
     _NODISCARD _CONSTEXPR20 size_type find_first_not_of(
         const basic_string& _Right, const size_type _Off = 0) const noexcept {
         // look for none of _Right at or after _Off
-        return static_cast<size_type>(_Traits_find_first_not_of<_Traits>(_Mypair._Myval2._Myptr(),
+        return _Find_result_cast(_Traits_find_first_not_of<_Traits>(_Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _Right._Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Right._Mypair._Myval2._Mysize)));
     }
@@ -2691,7 +2702,7 @@ public:
     _NODISCARD _CONSTEXPR20 size_type find_first_not_of(_In_reads_(_Count) const _Elem* const _Ptr,
         const size_type _Off, const size_type _Count) const noexcept /* strengthened */ {
         // look for none of [_Ptr, _Ptr + _Count) at or after _Off
-        return static_cast<size_type>(
+        return _Find_result_cast(
             _Traits_find_first_not_of<_Traits>(_Mypair._Myval2._Myptr(), static_cast<size_t>(_Mypair._Myval2._Mysize),
                 static_cast<size_t>(_Off), _Ptr, static_cast<size_t>(_Count)));
     }
@@ -2699,13 +2710,13 @@ public:
     _NODISCARD _CONSTEXPR20 size_type find_first_not_of(
         _In_z_ const _Elem* const _Ptr, size_type _Off = 0) const noexcept /* strengthened */ {
         // look for one of [_Ptr, <null>) at or after _Off
-        return static_cast<size_type>(_Traits_find_first_not_of<_Traits>(_Mypair._Myval2._Myptr(),
+        return _Find_result_cast(_Traits_find_first_not_of<_Traits>(_Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _Ptr, _Traits::length(_Ptr)));
     }
 
     _NODISCARD _CONSTEXPR20 size_type find_first_not_of(const _Elem _Ch, const size_type _Off = 0) const noexcept {
         // look for non-_Ch at or after _Off
-        return static_cast<size_type>(_Traits_find_not_ch<_Traits>(
+        return _Find_result_cast(_Traits_find_not_ch<_Traits>(
             _Mypair._Myval2._Myptr(), static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _Ch));
     }
 
@@ -2715,7 +2726,7 @@ public:
         noexcept(_Is_nothrow_convertible_v<const _StringViewIsh&, basic_string_view<_Elem, _Traits>>) {
         // look for none of _Right before _Off
         basic_string_view<_Elem, _Traits> _As_view = _Right;
-        return static_cast<size_type>(_Traits_find_last_not_of<_Traits>(_Mypair._Myval2._Myptr(),
+        return _Find_result_cast(_Traits_find_last_not_of<_Traits>(_Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _As_view.data(), _As_view.size()));
     }
 #endif // _HAS_CXX17
@@ -2723,7 +2734,7 @@ public:
     _NODISCARD _CONSTEXPR20 size_type find_last_not_of(
         const basic_string& _Right, const size_type _Off = npos) const noexcept {
         // look for none of _Right before _Off
-        return static_cast<size_type>(_Traits_find_last_not_of<_Traits>(_Mypair._Myval2._Myptr(),
+        return _Find_result_cast(_Traits_find_last_not_of<_Traits>(_Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _Right._Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Right._Mypair._Myval2._Mysize)));
     }
@@ -2731,7 +2742,7 @@ public:
     _NODISCARD _CONSTEXPR20 size_type find_last_not_of(_In_reads_(_Count) const _Elem* const _Ptr, const size_type _Off,
         const size_type _Count) const noexcept /* strengthened */ {
         // look for none of [_Ptr, _Ptr + _Count) before _Off
-        return static_cast<size_type>(
+        return _Find_result_cast(
             _Traits_find_last_not_of<_Traits>(_Mypair._Myval2._Myptr(), static_cast<size_t>(_Mypair._Myval2._Mysize),
                 static_cast<size_t>(_Off), _Ptr, static_cast<size_t>(_Count)));
     }
@@ -2739,13 +2750,13 @@ public:
     _NODISCARD _CONSTEXPR20 size_type find_last_not_of(
         _In_z_ const _Elem* const _Ptr, const size_type _Off = npos) const noexcept /* strengthened */ {
         // look for none of [_Ptr, <null>) before _Off
-        return static_cast<size_type>(_Traits_find_last_not_of<_Traits>(_Mypair._Myval2._Myptr(),
+        return _Find_result_cast(_Traits_find_last_not_of<_Traits>(_Mypair._Myval2._Myptr(),
             static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _Ptr, _Traits::length(_Ptr)));
     }
 
     _NODISCARD _CONSTEXPR20 size_type find_last_not_of(const _Elem _Ch, const size_type _Off = npos) const noexcept {
         // look for non-_Ch before _Off
-        return static_cast<size_type>(_Traits_rfind_not_ch<_Traits>(
+        return _Find_result_cast(_Traits_rfind_not_ch<_Traits>(
             _Mypair._Myval2._Myptr(), static_cast<size_t>(_Mypair._Myval2._Mysize), static_cast<size_t>(_Off), _Ch));
     }
 
diff --git a/tests/std/tests/GH_005546_containers_size_type_cast/test.cpp b/tests/std/tests/GH_005546_containers_size_type_cast/test.cpp
index 5116ee9ac55..ab5f5f84564 100644
--- a/tests/std/tests/GH_005546_containers_size_type_cast/test.cpp
+++ b/tests/std/tests/GH_005546_containers_size_type_cast/test.cpp
@@ -517,10 +517,87 @@ CONSTEXPR20 bool test() {
     return true;
 }
 
-#if _HAS_CXX20
-static_assert(test());
-#endif // _HAS_CXX20
+template <typename T = char>
+struct WideSizeAllocator {
+    using value_type = T;
+    using size_type  = unsigned long long;
+
+    WideSizeAllocator() = default;
+
+    template <typename U>
+    constexpr WideSizeAllocator(const WideSizeAllocator<U>&) noexcept {}
+
+    constexpr T* allocate(size_type n) {
+        return allocator<T>{}.allocate(static_cast<size_t>(n));
+    }
+    constexpr void deallocate(T* p, size_type n) noexcept {
+        allocator<T>{}.deallocate(p, static_cast<size_t>(n));
+    }
+};
+
+CONSTEXPR20 bool test_LWG4259() {
+    // On 32-bit platforms, basic_string may use 64-bit size_type (depending on the allocator), while basic_string_view
+    // uses 32-bit size_t. This makes basic_string_view::npos smaller than basic_string::npos.
+    using WideSizeString = basic_string<char, char_traits<char>, WideSizeAllocator<char>>;
+
+    WideSizeString str           = "Hello World";
+    WideSizeString not_found_str = "XYZ";
+    const char* not_found_cstr   = "XYZ";
+    assert(str.find(not_found_str) == WideSizeString::npos);
+    assert(str.find(not_found_cstr, 0, 3) == WideSizeString::npos);
+    assert(str.find(not_found_cstr) == WideSizeString::npos);
+    assert(str.find('Z') == WideSizeString::npos);
+#if _HAS_CXX17
+    string_view sv = "XYZ";
+    assert(str.find(sv) == WideSizeString::npos);
+#endif
+    assert(str.rfind(not_found_str) == WideSizeString::npos);
+    assert(str.rfind(not_found_cstr, WideSizeString::npos, 3) == WideSizeString::npos);
+    assert(str.rfind(not_found_cstr) == WideSizeString::npos);
+    assert(str.rfind('Z') == WideSizeString::npos);
+#if _HAS_CXX17
+    assert(str.rfind(sv) == WideSizeString::npos);
+#endif
+    assert(str.find_first_of(not_found_str) == WideSizeString::npos);
+    assert(str.find_first_of(not_found_cstr, 0, 3) == WideSizeString::npos);
+    assert(str.find_first_of(not_found_cstr) == WideSizeString::npos);
+    assert(str.find_first_of('Z') == WideSizeString::npos);
+#if _HAS_CXX17
+    assert(str.find_first_of(sv) == WideSizeString::npos);
+#endif
+    WideSizeString all_in_str = "Hello World!";
+    WideSizeString search_all = "Hello World";
+    assert(search_all.find_first_not_of(all_in_str) == WideSizeString::npos);
+    assert(search_all.find_first_not_of("Hello World!", 0, 12) == WideSizeString::npos);
+    assert(search_all.find_first_not_of("Hello World!") == WideSizeString::npos);
+    WideSizeString repeated = "AAAAA";
+    assert(repeated.find_first_not_of('A') == WideSizeString::npos);
+#if _HAS_CXX17
+    string_view sv_all = "Hello World!";
+    assert(search_all.find_first_not_of(sv_all) == WideSizeString::npos);
+#endif
+    assert(str.find_last_of(not_found_str) == WideSizeString::npos);
+    assert(str.find_last_of(not_found_cstr, WideSizeString::npos, 3) == WideSizeString::npos);
+    assert(str.find_last_of(not_found_cstr) == WideSizeString::npos);
+    assert(str.find_last_of('Z') == WideSizeString::npos);
+#if _HAS_CXX17
+    assert(str.find_last_of(sv) == WideSizeString::npos);
+#endif
+    assert(search_all.find_last_not_of(all_in_str) == WideSizeString::npos);
+    assert(search_all.find_last_not_of("Hello World!", WideSizeString::npos, 12) == WideSizeString::npos);
+    assert(search_all.find_last_not_of("Hello World!") == WideSizeString::npos);
+    assert(repeated.find_last_not_of('A') == WideSizeString::npos);
+#if _HAS_CXX17
+    assert(search_all.find_last_not_of(sv_all) == WideSizeString::npos);
+#endif
+    return true;
+}
 
 int main() {
+#if _HAS_CXX20
+    static_assert(test());
+    static_assert(test_LWG4259());
+#endif // _HAS_CXX20
     test();
+    test_LWG4259();
 }