[libc++] Add a test for PR14074

PR14074 was fixed in r165884, but no tests were added.

llvm-svn: 354943

libcxx/test/std/input.output/stream.buffers/streambuf/streambuf.virtuals/streambuf.virt.put/xsputn.PR14074.pass.cpp

@@ -0,0 +1,62 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <streambuf>
+
+// template <class charT, class traits = char_traits<charT> >
+// class basic_streambuf;
+
+// streamsize xsputn(const char_type* s, streamsize n);
+
+// Test https://bugs.llvm.org/show_bug.cgi?id=14074. The bug is really inside
+// basic_streambuf, but I can't seem to reproduce without going through one
+// of its derived classes.
+
+#include <cassert>
+#include <cstddef>
+#include <cstdio>
+#include <fstream>
+#include <sstream>
+#include <string>
+#include "platform_support.h"
+
+
+// Count the number of bytes in a file -- make sure to use only functionality
+// provided by the C library to avoid relying on the C++ library, which we're
+// trying to test.
+static std::size_t count_bytes(char const* filename) {
+    std::FILE* f = std::fopen(filename, "rb");
+    std::size_t count = 0;
+    while (std::fgetc(f) != EOF)
+        ++count;
+    return count;
+}
+
+int main(int, char**) {
+    {
+        // with basic_stringbuf
+        std::basic_stringbuf<char> buf;
+        std::streamsize sz = buf.sputn("\xFF", 1);
+        assert(sz == 1);
+        assert(buf.str().size() == 1);
+    }
+    {
+        // with basic_filebuf
+        std::string temp = get_temp_file_name();
+        {
+            std::basic_filebuf<char> buf;
+            buf.open(temp.c_str(), std::ios_base::out);
+            std::streamsize sz = buf.sputn("\xFF", 1);
+            assert(sz == 1);
+        }
+        assert(count_bytes(temp.c_str()) == 1);
+        std::remove(temp.c_str());
+    }
+
+    return 0;
+}