Add diagnostics for min/max algorithms when a InputIterator is used.

These algorithms require a ForwardIterator or better. Ensure
we diagnose the contract violation at compile time instead of
of silently doing the wrong thing.

Further algorithms will be audited in upcoming patches.

llvm-svn: 340426

2 files changed, 43 insertions, 0 deletions

diff --git a/libcxx/include/algorithm b/libcxx/include/algorithm
index 74a326dbf5e..ee2a54d7232 100644
--- a/libcxx/include/algorithm
+++ b/libcxx/include/algorithm
@@ -2398,6 +2398,8 @@ inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
 _ForwardIterator
 min_element(_ForwardIterator __first, _ForwardIterator __last, _Compare __comp)
 {
+    static_assert(__is_forward_iterator<_ForwardIterator>::value,
+        "std::min_element requires a ForwardIterator");
     if (__first != __last)
     {
         _ForwardIterator __i = __first;
@@ -2462,6 +2464,8 @@ inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
 _ForwardIterator
 max_element(_ForwardIterator __first, _ForwardIterator __last, _Compare __comp)
 {
+    static_assert(__is_forward_iterator<_ForwardIterator>::value,
+        "std::max_element requires a ForwardIterator");
     if (__first != __last)
     {
         _ForwardIterator __i = __first;
@@ -2548,6 +2552,8 @@ _LIBCPP_CONSTEXPR_AFTER_CXX11
 std::pair<_ForwardIterator, _ForwardIterator>
 minmax_element(_ForwardIterator __first, _ForwardIterator __last, _Compare __comp)
 {
+  static_assert(__is_forward_iterator<_ForwardIterator>::value,
+        "std::minmax_element requires a ForwardIterator");
   std::pair<_ForwardIterator, _ForwardIterator> __result(__first, __first);
   if (__first != __last)
   {
diff --git a/libcxx/test/std/algorithms/alg.sorting/alg.min.max/requires_forward_iterator.fail.cpp b/libcxx/test/std/algorithms/alg.sorting/alg.min.max/requires_forward_iterator.fail.cpp
new file mode 100644
index 00000000000..a1069dee7ae
--- /dev/null
+++ b/libcxx/test/std/algorithms/alg.sorting/alg.min.max/requires_forward_iterator.fail.cpp
@@ -0,0 +1,37 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <algorithm>
+
+// template<ForwardIterator Iter>
+//   max_element(Iter first, Iter last);
+
+#include <algorithm>
+#include <cassert>
+
+#include "test_iterators.h"
+
+int main() {
+  int arr[] = {1, 2, 3};
+  const int *b = std::begin(arr), *e = std::end(arr);
+  typedef input_iterator<const int*> Iter;
+  {
+    // expected-error@algorithm:* {{"std::min_element requires a ForwardIterator"}}
+    std::min_element(Iter(b), Iter(e));
+  }
+  {
+    // expected-error@algorithm:* {{"std::max_element requires a ForwardIterator"}}
+    std::max_element(Iter(b), Iter(e));
+  }
+  {
+    // expected-error@algorithm:* {{"std::minmax_element requires a ForwardIterator"}}
+    std::minmax_element(Iter(b), Iter(e));
+  }
+
+}