Implement LWG 2711. Constrain path members.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@279945 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/experimental/filesystem/class.path/path.member/path.append.pass.cpp

@@ -246,6 +246,60 @@ void doAppendSourceTest(AppendOperatorTestcase const& TC)
   }
 }
 
+
+
+template <class It, class = decltype(fs::path{}.append(std::declval<It>()))>
+constexpr bool has_append(int) { return true; }
+template <class It>
+constexpr bool has_append(long) { return false; }
+
+template <class It, class = decltype(fs::path{}.operator/=(std::declval<It>()))>
+constexpr bool has_append_op(int) { return true; }
+template <class It>
+constexpr bool has_append_op(long) { return false; }
+
+template <class It>
+constexpr bool has_append() {
+  static_assert(has_append<It>(0) == has_append_op<It>(0), "must be same");
+  return has_append<It>(0) && has_append_op<It>(0);
+}
+
+void test_sfinae()
+{
+  using namespace fs;
+  {
+    using It = const char* const;
+    static_assert(has_append<It>(), "");
+  }
+  {
+    using It = input_iterator<const char*>;
+    static_assert(has_append<It>(), "");
+  }
+  {
+    struct Traits {
+      using iterator_category = std::input_iterator_tag;
+      using value_type = const char;
+      using pointer = const char*;
+      using reference = const char&;
+      using difference_type = std::ptrdiff_t;
+    };
+    using It = input_iterator<const char*, Traits>;
+    static_assert(has_append<It>(), "");
+  }
+  {
+    using It = output_iterator<const char*>;
+    static_assert(!has_append<It>(), "");
+
+  }
+  {
+    static_assert(!has_append<int*>(), "");
+  }
+  {
+    static_assert(!has_append<char>(), "");
+    static_assert(!has_append<const char>(), "");
+  }
+}
+
 int main()
 {
   using namespace fs;
@@ -266,4 +320,5 @@ int main()
     doAppendSourceAllocTest<char>(TC);
     doAppendSourceAllocTest<wchar_t>(TC);
   }
+  test_sfinae();
 }