Set the buffer of an fstream to empty when the underlying file is closed. This 'fixes' PR#38052 - std::fstream still good after closing and updating content.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@350603 91177308-0d34-0410-b5e6-96231b3b80d8

include/fstream

@@ -702,6 +702,7 @@ basic_filebuf<_CharT, _Traits>::close()
             __file_ = 0;
         else
             __rt = 0;
+        setbuf(0, 0);
     }
     return __rt;
 }

test/libcxx/input.output/file.streams/fstreams/fstream.close.pass.cpp

@@ -0,0 +1,35 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+// <fstream>
+
+// template <class charT, class traits = char_traits<charT> >
+// class basic_fstream
+
+// close();
+
+//	Inspired by PR#38052 - std::fstream still good after closing and updating content
+
+#include <fstream>
+#include <cassert>
+#include "platform_support.h"
+
+int main()
+{
+    std::string temp = get_temp_file_name();
+
+    std::fstream ofs(temp, std::ios::out | std::ios::trunc);
+    ofs << "Hello, World!\n";
+    assert( ofs.good());
+    ofs.close();
+    assert( ofs.good());
+    ofs << "Hello, World!\n";
+    assert(!ofs.good());
+
+    std::remove(temp.c_str());
+}