[libcxx] [test] Portability fix of std::basic_filebuf::overflow() test.

[amakc11]

Summary

The standard specifies the behavior of std::basic_filebuf::overflow() as follows: "Behaves according to the description of basic_­streambuf<charT, traits>​::​overflow(c), except that the behavior of “consuming characters” is performed by first converting". The behavior of basic_­streambuf<charT, traits>​::​overflow(c) is specified as follows: "Let r be the number of characters in the pending sequence not consumed. If r is nonzero then pbase() and pptr() are set so that: pptr() - pbase() == r and the r characters starting at pbase() are the associated output stream. In case r is zero (all characters of the pending sequence have been consumed) then either pbase() is set to nullptr, or pbase() and pptr() are both set to the same non-null value". It means that in general the post-condition of this method is (pptr() == pbase()) || (pbase() == nullptr) || (pptr() - pbase() == r). This patch adds this post-condition check and marks other checks as libc++ specific making the test more portable.

BTW, methods pptr(), pbase() and epptr() return pointers, but this test compares their results with zero instead of nullptr. This is also fixed.

[amakc11]

The first parameter of pubsetbuf method is also a pointer, so zero is changed to` nullptr`. Missed this in the original patch.

libcxx/test/std/input.output/file.streams/fstreams/filebuf.virtuals/overflow.pass.cpp

 };
 
 int main(int, char**)
 {
     {
         test_buf<char> f;
         assert(f.open("overflow.dat", std::ios_base::out) != 0);
         assert(f.is_open());
-        assert(f.pbase() == 0);
-        assert(f.pptr() == 0);
-        assert(f.epptr() == 0);
+        assert(f.pbase() == nullptr);
+        assert(f.pptr()  == nullptr);
+        assert(f.epptr() == nullptr);
         assert(f.overflow('a') == 'a');
-        assert(f.pbase() != 0);
-        assert(f.pptr() == f.pbase());
-        assert(f.epptr() - f.pbase() == 4095);
+        assert((f.pptr() == f.pbase()) || (f.pbase() == nullptr) || (f.pptr() - f.pbase() == 1));
+        LIBCPP_ASSERT(f.pbase() != nullptr);
+        LIBCPP_ASSERT(f.pptr()  == f.pbase());
+        LIBCPP_ASSERT(f.epptr() - f.pbase() == 4095);
     }
     {
         test_buf<char> f;
         assert(f.open("overflow.dat", std::ios_base::in) != 0);
         assert(f.is_open());
         assert(f.sgetc() == 'a');
     }
     std::remove("overflow.dat");
     {
         test_buf<char> f;
-        f.pubsetbuf(0, 0);
+        f.pubsetbuf(nullptr, 0);
         assert(f.open("overflow.dat", std::ios_base::out) != 0);
         assert(f.is_open());
-        assert(f.pbase() == 0);
-        assert(f.pptr() == 0);
-        assert(f.epptr() == 0);
+        assert(f.pbase() == nullptr);
+        assert(f.pptr()  == nullptr);
+        assert(f.epptr() == nullptr);
         assert(f.overflow('a') == 'a');
-        assert(f.pbase() == 0);
-        assert(f.pptr() == 0);
-        assert(f.epptr() == 0);
+        assert(f.pbase() == nullptr);
+        assert(f.pptr()  == nullptr);
+        assert(f.epptr() == nullptr);
     }
     {
         test_buf<char> f;
         assert(f.open("overflow.dat", std::ios_base::in) != 0);
         assert(f.is_open());
         assert(f.sgetc() == 'a');
     }
     std::remove("overflow.dat");
     {
         test_buf<wchar_t> f;
         assert(f.open("overflow.dat", std::ios_base::out) != 0);
         assert(f.is_open());
-        assert(f.pbase() == 0);
-        assert(f.pptr() == 0);
-        assert(f.epptr() == 0);
+        assert(f.pbase() == nullptr);
+        assert(f.pptr()  == nullptr);
+        assert(f.epptr() == nullptr);
         assert(f.overflow(L'a') == L'a');
-        assert(f.pbase() != 0);
-        assert(f.pptr() == f.pbase());
-        assert(f.epptr() - f.pbase() == 4095);
+        assert((f.pptr() == f.pbase()) || (f.pbase() == nullptr) || (f.pptr() - f.pbase() == 1));
+        LIBCPP_ASSERT(f.pbase() != nullptr);
+        LIBCPP_ASSERT(f.pptr()  == f.pbase());
+        LIBCPP_ASSERT(f.epptr() - f.pbase() == 4095);
     }
     {
         test_buf<wchar_t> f;
         assert(f.open("overflow.dat", std::ios_base::in) != 0);
         assert(f.is_open());
         assert(f.sgetc() == L'a');
     }
     std::remove("overflow.dat");
     {
         test_buf<wchar_t> f;
-        f.pubsetbuf(0, 0);
+        f.pubsetbuf(nullptr, 0);
         assert(f.open("overflow.dat", std::ios_base::out) != 0);
         assert(f.is_open());
-        assert(f.pbase() == 0);
-        assert(f.pptr() == 0);
-        assert(f.epptr() == 0);
+        assert(f.pbase() == nullptr);
+        assert(f.pptr()  == nullptr);
+        assert(f.epptr() == nullptr);
         assert(f.overflow(L'a') == L'a');
-        assert(f.pbase() == 0);
-        assert(f.pptr() == 0);
-        assert(f.epptr() == 0);
+        assert(f.pbase() == nullptr);
+        assert(f.pptr()  == nullptr);
+        assert(f.epptr() == nullptr);
     }
     {
         test_buf<wchar_t> f;
         assert(f.open("overflow.dat", std::ios_base::in) != 0);
         assert(f.is_open());
         assert(f.sgetc() == L'a');
     }
     std::remove("overflow.dat");