address code review

Author: huixie90

libcxx/include/__iterator/prev.h

@@ -39,10 +39,10 @@ prev(_InputIter __x, typename iterator_traits<_InputIter>::difference_type __n)
   return __x;
 }
 
-template <class _BidirectionalIterator,
-          __enable_if_t<__has_bidirectional_iterator_category<_BidirectionalIterator>::value, int> = 0>
-[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 _BidirectionalIterator
-prev(_BidirectionalIterator __it) {
+template <class _InputIter, __enable_if_t<__has_input_iterator_category<_InputIter>::value, int> = 0>
+[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 _InputIter prev(_InputIter __it) {
+  static_assert(__has_bidirectional_iterator_category<_InputIter>::value,
+                "Attempt to prev(it) with a non-bidirectional iterator");
   return std::prev(std::move(__it), 1);
 }
 

libcxx/test/libcxx/iterators/iterator.primitives/iterator.operations/prev.verify.cpp

@@ -0,0 +1,19 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// std::prev
+
+#include <iterator>
+#include "test_iterators.h"
+
+void test() {
+  int arr[] = {1, 2};
+  cpp17_input_iterator<int*> it(&arr[0]);
+  it = std::prev(it);
+  // expected-error-re@*:* {{static assertion failed due to requirement {{.*}}: Attempt to prev(it) with a non-bidirectional iterator}}
+}

libcxx/test/std/iterators/iterator.primitives/iterator.operations/prev.pass.cpp

@@ -18,21 +18,6 @@
 #include "test_macros.h"
 #include "test_iterators.h"
 
-template <class Iter>
-std::false_type prev_test(...);
-
-template <class Iter>
-decltype((void)std::prev(std::declval<Iter>()), std::true_type()) prev_test(int);
-
-template <class Iter>
-using CanPrev = decltype(prev_test<Iter>(0));
-
-static_assert(!CanPrev<cpp17_input_iterator<int*> >::value, "");
-static_assert(CanPrev<bidirectional_iterator<int*> >::value, "");
-#if TEST_STD_VER >= 20
-static_assert(!CanPrev<cpp20_random_access_iterator<int*> >::value);
-#endif
-
 template <class It>
 TEST_CONSTEXPR_CXX17 void
 check_prev_n(It it, typename std::iterator_traits<It>::difference_type n, It result)