Index: libcxx/include/fstream
===================================================================
--- libcxx/include/fstream
+++ libcxx/include/fstream
@@ -425,19 +425,33 @@
     }
     else
     {
-        ptrdiff_t __ln = __extbufnext_ - __extbuf_;
-        ptrdiff_t __le = __extbufend_ - __extbuf_;
-        ptrdiff_t __rn = __rhs.__extbufnext_ - __rhs.__extbuf_;
-        ptrdiff_t __re = __rhs.__extbufend_ - __rhs.__extbuf_;
+        ptrdiff_t __ln = 0, __le = 0, __rn = 0, __re = 0;
+        if (__extbufnext_ != nullptr)
+            __ln = __extbufnext_ - __extbuf_;
+        if (__extbufend_ != nullptr)
+            __le = __extbufend_ - __extbuf_;
+        if (__rhs.__extbufnext_ != nullptr)
+            __rn = __rhs.__extbufnext_ - __rhs.__extbuf_;
+        if (__rhs.__extbufend_ != nullptr)
+            __re = __rhs.__extbufend_ - __rhs.__extbuf_;
         if (__extbuf_ == __extbuf_min_ && __rhs.__extbuf_ != __rhs.__extbuf_min_)
         {
             __extbuf_ = __rhs.__extbuf_;
             __rhs.__extbuf_ = __rhs.__extbuf_min_;
+            _VSTD::memmove(__rhs.__extbuf_min_, __extbuf_min_, sizeof(__extbuf_min_));
         }
         else if (__extbuf_ != __extbuf_min_ && __rhs.__extbuf_ == __rhs.__extbuf_min_)
         {
             __rhs.__extbuf_ = __extbuf_;
             __extbuf_ = __extbuf_min_;
+            _VSTD::memmove(__extbuf_min_, __rhs.__extbuf_min_, sizeof(__extbuf_min_));
+        }
+        else
+        {
+            char __tmp[sizeof(__extbuf_min_)];
+            _VSTD::memmove(__tmp, __extbuf_min_, sizeof(__extbuf_min_));
+            _VSTD::memmove(__extbuf_min_, __rhs.__extbuf_min_, sizeof(__extbuf_min_));
+            _VSTD::memmove(__rhs.__extbuf_min_, __tmp, sizeof(__extbuf_min_));
         }
         __extbufnext_ = __extbuf_ + __rn;
         __extbufend_ = __extbuf_ + __re;
Index: libcxx/test/std/input.output/file.streams/fstreams/filebuf.assign/nonmember_swap_min.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/std/input.output/file.streams/fstreams/filebuf.assign/nonmember_swap_min.pass.cpp
@@ -0,0 +1,61 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <fstream>
+
+// template <class charT, class traits = char_traits<charT> >
+// class basic_filebuf
+
+// template <class charT, class traits>
+// void
+// swap(basic_filebuf<charT, traits>& x, basic_filebuf<charT, traits>& y);
+
+#include <fstream>
+#include <cassert>
+#include "test_macros.h"
+#include "platform_support.h"
+
+int main(int, char**)
+{
+    std::string tmp = get_temp_file_name();
+    std::string tmpA = get_temp_file_name();
+    std::string tmpB = get_temp_file_name();
+
+    {
+        std::ofstream sa(tmpA), sb(tmpB);
+        sa << "AAAA";
+        sb << "BBBB";
+    }
+
+    std::filebuf f1;
+    assert(f1.open(tmpA, std::ios_base::in) != 0);
+    assert(f1.is_open());
+    f1.pubsetbuf(0, 0);
+
+    std::filebuf f2;
+    assert(f2.open(tmpB, std::ios_base::in) != 0);
+    assert(f2.is_open());
+    f2.pubsetbuf(0, 0);
+
+    assert(f1.sgetc() == 'A');
+    assert(f2.sgetc() == 'B');
+
+    swap(f1, f2);
+
+    assert(f1.is_open());
+    assert(f2.is_open());
+
+    assert(f1.sgetc() == 'B');
+    assert(f2.sgetc() == 'A');
+
+    std::remove(tmp.c_str());
+    std::remove(tmpA.c_str());
+    std::remove(tmpB.c_str());
+
+    return 0;
+}