[libc++] Don't require complete types in vector<T>::empty()

libcxx/include/__vector/layout.h

@@ -199,6 +199,7 @@ class __vector_layout {
 
   [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI size_type __size() const _NOEXCEPT;
   [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI size_type __capacity() const _NOEXCEPT;
+  [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI bool __empty() const _NOEXCEPT;
   [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI pointer __end_ptr() _NOEXCEPT;
   [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI const_pointer __end_ptr() const _NOEXCEPT;
   [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI pointer __capacity_ptr() _NOEXCEPT;
@@ -313,6 +314,11 @@ __vector_layout<_Tp, _Alloc>::__capacity() const _NOEXCEPT {
   return __capacity_;
 }
 
+template <class _Tp, class _Alloc>
+_LIBCPP_CONSTEXPR_SINCE_CXX20 bool __vector_layout<_Tp, _Alloc>::__empty() const _NOEXCEPT {
+  return __size_ == 0;
+}
+
 template <class _Tp, class _Alloc>
 _LIBCPP_CONSTEXPR_SINCE_CXX20 typename __vector_layout<_Tp, _Alloc>::pointer
 __vector_layout<_Tp, _Alloc>::__end_ptr() _NOEXCEPT {
@@ -425,6 +431,11 @@ __vector_layout<_Tp, _Alloc>::__capacity() const _NOEXCEPT {
   return static_cast<size_type>(__capacity_ - __begin_);
 }
 
+template <class _Tp, class _Alloc>
+_LIBCPP_CONSTEXPR_SINCE_CXX20 bool __vector_layout<_Tp, _Alloc>::__empty() const _NOEXCEPT {
+  return __begin_ == __end_;
+}
+
 template <class _Tp, class _Alloc>
 _LIBCPP_CONSTEXPR_SINCE_CXX20 typename __vector_layout<_Tp, _Alloc>::pointer
 __vector_layout<_Tp, _Alloc>::__end_ptr() _NOEXCEPT {

libcxx/include/__vector/vector.h

@@ -398,7 +398,7 @@ class vector {
     return __layout_.__capacity();
   }
   [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI bool empty() const _NOEXCEPT {
-    return size() == 0;
+    return __layout_.__empty();
   }
 
   [[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI size_type max_size() const _NOEXCEPT {

libcxx/test/libcxx/containers/sequences/vector/incomplete_type.compile.pass.cpp

@@ -0,0 +1,27 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <vector>
+
+// This test pins down the current libc++ behavior that vector<T>::empty() can be
+// called even when T is an incomplete type. The standard does not require this:
+// [vector.overview] only guarantees that an incomplete type may be used to
+// instantiate vector, and requires the type to be complete before any method is
+// called.
+//
+// However, libc++ made that work previously, and this test pins down that behavior
+// to avoid breaking it unintentionally. Note that this is not a guarantee to users
+// that we will support this in the future: this merely guards against changing this
+// behavior unknowingly.
+
+#include <vector>
+
+struct Incomplete;
+
+bool call_empty(std::vector<Incomplete>& v) { return v.empty(); }
+bool call_empty_const(const std::vector<Incomplete>& v) { return v.empty(); }