Add test case for xsgetn in basic_filebuf

This is the promised follow-up to #167779. It simply adds a test case.

Author: Sterling-Augustine

libcxx/test/std/input.output/file.streams/fstreams/ifstream.members/xsgetn.pass.cpp

@@ -0,0 +1,72 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// FILE_DEPENDENCIES: xsgetn.test.dat
+
+// <fstream>
+
+// template <class charT, class traits = char_traits<charT> >
+// class basic_ifstream
+
+// streamsize xsgetn(char_type*, streamsize) override;
+
+// This isn't a required override by the standard, but most implementations override it, since it allows for
+// significantly improved performance in some cases. All of this code is required to work, so this isn't a libc++
+// extension
+
+#include <cassert>
+#include <fstream>
+
+#include "test_macros.h"
+
+int main(int, char**) {
+  {
+    char buffer[10];
+    std::ifstream fs("xsgetn.test.dat");
+    std::filebuf* fb = fs.rdbuf();
+    fb->pubsetbuf(buffer, 10);
+
+    // Ensure that the buffer is set up
+    assert(fb->sgetc() == 't');
+
+    std::string str(5, '\0');
+
+    { // Check that a read smaller than the buffer works fine
+      assert(fb->sgetn(str.data(), 5) == 5);
+      assert(str == "this ");
+    }
+    { // Check that reading up to the buffer end works fine
+      assert(fb->sgetn(str.data(), 5) == 5);
+      assert(str == "is so");
+    }
+    { // Check that reading from an empty buffer, but more than the buffer can hold works fine
+      str.resize(12);
+      assert(fb->sgetn(str.data(), 12) == 12);
+      assert(str == "me random da");
+    }
+    { // Check that reading from a non-empty buffer, and more than the buffer can hold works fine
+      // Fill the buffer up
+      str.resize(2);
+      assert(fb->sgetn(str.data(), 2) == 2);
+      assert(str == "ta");
+
+      // Do the actual check
+      str.resize(12);
+      assert(fb->sgetn(str.data(), 12) == 12);
+      assert(str == " to be able ");
+    }
+    { // Check that trying to read more than the file size works fine
+      str.resize(30);
+      assert(fb->sgetn(str.data(), 30) == 24);
+      str.resize(24);
+      assert(str == "to test buffer behaviour");
+    }
+  }
+
+  return 0;
+}

libcxx/test/std/input.output/file.streams/fstreams/ifstream.members/xsgetn.test.dat

@@ -0,0 +1 @@
+this is some random data to be able to test buffer behaviour