Avoid violating precondition of __bit_log2

Author: frederick-vs-ja

libcxx/include/__algorithm/sort.h

@@ -904,6 +904,9 @@ template <class _AlgPolicy, class _Type, __enable_if_t<__sort_is_specialized_in_
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __sort_dispatch(_Type* __first, _Type* __last, __less<>&) {
 #if _LIBCPP_STD_VER >= 20
   if (std::is_constant_evaluated()) {
+    if (__last == first)
+      return;
+
     auto __depth_limit = static_cast<ptrdiff_t>(2 * std::__bit_log2(static_cast<size_t>(__last - __first)));
     std::__introsort<_ClassicAlgPolicy, ranges::less, _Type*, __use_branchless_sort<ranges::less, _Type*>>(
         __first, __last, ranges::less{}, __depth_limit);
@@ -919,6 +922,9 @@ template <class _AlgPolicy, class _Type, __enable_if_t<__sort_is_specialized_in_
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __sort_dispatch(_Type* __first, _Type* __last, less<_Type>&) {
 #if _LIBCPP_STD_VER >= 20
   if (std::is_constant_evaluated()) {
+    if (__last == first)
+      return;
+
     auto __depth_limit = static_cast<ptrdiff_t>(2 * std::__bit_log2(static_cast<size_t>(__last - __first)));
     std::__introsort<_ClassicAlgPolicy, ranges::less, _Type*, __use_branchless_sort<ranges::less, _Type*>>(
         __first, __last, ranges::less{}, __depth_limit);
@@ -935,6 +941,9 @@ template <class _AlgPolicy, class _Type, __enable_if_t<__sort_is_specialized_in_
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __sort_dispatch(_Type* __first, _Type* __last, less<>&) {
 #  if _LIBCPP_STD_VER >= 20
   if (std::is_constant_evaluated()) {
+    if (__last == first)
+      return;
+
     auto __depth_limit = static_cast<ptrdiff_t>(2 * std::__bit_log2(static_cast<size_t>(__last - __first)));
     std::__introsort<_ClassicAlgPolicy, ranges::less, _Type*, __use_branchless_sort<ranges::less, _Type*>>(
         __first, __last, ranges::less{}, __depth_limit);
@@ -951,6 +960,9 @@ _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __sort_dispatch(_Type*
 template <class _AlgPolicy, class _Type, __enable_if_t<__sort_is_specialized_in_library<_Type>::value, int> = 0>
 _LIBCPP_HIDE_FROM_ABI constexpr void __sort_dispatch(_Type* __first, _Type* __last, ranges::less&) {
   if (std::is_constant_evaluated()) {
+    if (__last == first)
+      return;
+
     auto __depth_limit = static_cast<ptrdiff_t>(2 * std::__bit_log2(static_cast<size_t>(__last - __first)));
     std::__introsort<_ClassicAlgPolicy, ranges::less, _Type*, __use_branchless_sort<ranges::less, _Type*>>(
         __first, __last, ranges::less{}, __depth_limit);