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.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@340426 91177308-0d34-0410-b5e6-96231b3b80d8

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));
+  }
+
+}