diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -385,11 +385,17 @@
   __functional/unwrap_ref.h
   __functional/weak_result_type.h
   __fwd/array.h
+  __fwd/fstream.h
   __fwd/get.h
   __fwd/hash.h
+  __fwd/ios.h
+  __fwd/istream.h
   __fwd/memory_resource.h
+  __fwd/ostream.h
   __fwd/pair.h
   __fwd/span.h
+  __fwd/sstream.h
+  __fwd/streambuf.h
   __fwd/string.h
   __fwd/string_view.h
   __fwd/subrange.h
diff --git a/libcxx/include/__fwd/fstream.h b/libcxx/include/__fwd/fstream.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__fwd/fstream.h
@@ -0,0 +1,53 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___FWD_FSTREAM_H
+#define _LIBCPP___FWD_FSTREAM_H
+
+#include <__config>
+#include <__fwd/string.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _CharT, class _Traits = char_traits<_CharT> >
+    class _LIBCPP_TEMPLATE_VIS basic_filebuf;
+template <class _CharT, class _Traits = char_traits<_CharT> >
+    class _LIBCPP_TEMPLATE_VIS basic_ifstream;
+template <class _CharT, class _Traits = char_traits<_CharT> >
+    class _LIBCPP_TEMPLATE_VIS basic_ofstream;
+template <class _CharT, class _Traits = char_traits<_CharT> >
+    class _LIBCPP_TEMPLATE_VIS basic_fstream;
+
+typedef basic_filebuf<char>          filebuf;
+typedef basic_ifstream<char>         ifstream;
+typedef basic_ofstream<char>         ofstream;
+typedef basic_fstream<char>          fstream;
+
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+typedef basic_filebuf<wchar_t>       wfilebuf;
+typedef basic_ifstream<wchar_t>      wifstream;
+typedef basic_ofstream<wchar_t>      wofstream;
+typedef basic_fstream<wchar_t>       wfstream;
+#endif
+
+template <class _CharT, class _Traits>
+    class _LIBCPP_PREFERRED_NAME(filebuf) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wfilebuf)) basic_filebuf;
+template <class _CharT, class _Traits>
+    class _LIBCPP_PREFERRED_NAME(ifstream) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wifstream)) basic_ifstream;
+template <class _CharT, class _Traits>
+    class _LIBCPP_PREFERRED_NAME(ofstream) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wofstream)) basic_ofstream;
+template <class _CharT, class _Traits>
+    class _LIBCPP_PREFERRED_NAME(fstream) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wfstream)) basic_fstream;
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___FWD_FSTREAM_H
diff --git a/libcxx/include/__fwd/ios.h b/libcxx/include/__fwd/ios.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__fwd/ios.h
@@ -0,0 +1,41 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___FWD_IOS_H
+#define _LIBCPP___FWD_IOS_H
+
+#include <__config>
+#include <__fwd/string.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _CharT, class _Traits = char_traits<_CharT> >
+    class _LIBCPP_TEMPLATE_VIS basic_ios;
+
+typedef basic_ios<char>              ios;
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+typedef basic_ios<wchar_t>           wios;
+#endif
+
+template <class _CharT, class _Traits>
+    class _LIBCPP_PREFERRED_NAME(ios) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wios)) basic_ios;
+
+#if defined(_NEWLIB_VERSION)
+// On newlib, off_t is 'long int'
+typedef long int streamoff;         // for char_traits in <string>
+#else
+typedef long long streamoff;        // for char_traits in <string>
+#endif
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___FWD_IOS_H
diff --git a/libcxx/include/__fwd/istream.h b/libcxx/include/__fwd/istream.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__fwd/istream.h
@@ -0,0 +1,43 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___FWD_ISTREAM_H
+#define _LIBCPP___FWD_ISTREAM_H
+
+#include <__config>
+#include <__fwd/string.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _CharT, class _Traits = char_traits<_CharT> >
+    class _LIBCPP_TEMPLATE_VIS basic_istream;
+
+template <class _CharT, class _Traits = char_traits<_CharT> >
+    class _LIBCPP_TEMPLATE_VIS basic_iostream;
+
+typedef basic_istream<char>          istream;
+typedef basic_iostream<char>         iostream;
+
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+typedef basic_istream<wchar_t>       wistream;
+typedef basic_iostream<wchar_t>      wiostream;
+#endif
+
+template <class _CharT, class _Traits>
+    class _LIBCPP_PREFERRED_NAME(istream) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wistream)) basic_istream;
+
+template <class _CharT, class _Traits>
+    class _LIBCPP_PREFERRED_NAME(iostream) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wiostream)) basic_iostream;
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___FWD_ISTREAM_H
diff --git a/libcxx/include/__fwd/ostream.h b/libcxx/include/__fwd/ostream.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__fwd/ostream.h
@@ -0,0 +1,35 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___FWD_OSTREAM_H
+#define _LIBCPP___FWD_OSTREAM_H
+
+#include <__config>
+#include <__fwd/string.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _CharT, class _Traits = char_traits<_CharT> >
+    class _LIBCPP_TEMPLATE_VIS basic_ostream;
+
+typedef basic_ostream<char>          ostream;
+
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+typedef basic_ostream<wchar_t>       wostream;
+#endif
+
+template <class _CharT, class _Traits>
+    class _LIBCPP_PREFERRED_NAME(ostream) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wostream)) basic_ostream;
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___FWD_OSTREAM_H
diff --git a/libcxx/include/__fwd/sstream.h b/libcxx/include/__fwd/sstream.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__fwd/sstream.h
@@ -0,0 +1,58 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___FWD_SSTREAM_H
+#define _LIBCPP___FWD_SSTREAM_H
+
+#include <__config>
+#include <__fwd/string.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _CharT, class _Traits = char_traits<_CharT>,
+          class _Allocator = allocator<_CharT> >
+    class _LIBCPP_TEMPLATE_VIS basic_stringbuf;
+
+template <class _CharT, class _Traits = char_traits<_CharT>,
+          class _Allocator = allocator<_CharT> >
+    class _LIBCPP_TEMPLATE_VIS basic_istringstream;
+template <class _CharT, class _Traits = char_traits<_CharT>,
+          class _Allocator = allocator<_CharT> >
+    class _LIBCPP_TEMPLATE_VIS basic_ostringstream;
+template <class _CharT, class _Traits = char_traits<_CharT>,
+          class _Allocator = allocator<_CharT> >
+    class _LIBCPP_TEMPLATE_VIS basic_stringstream;
+
+typedef basic_stringbuf<char>        stringbuf;
+typedef basic_istringstream<char>    istringstream;
+typedef basic_ostringstream<char>    ostringstream;
+typedef basic_stringstream<char>     stringstream;
+
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+typedef basic_stringbuf<wchar_t>     wstringbuf;
+typedef basic_istringstream<wchar_t> wistringstream;
+typedef basic_ostringstream<wchar_t> wostringstream;
+typedef basic_stringstream<wchar_t>  wstringstream;
+#endif
+
+template <class _CharT, class _Traits, class _Allocator>
+    class _LIBCPP_PREFERRED_NAME(stringbuf) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wstringbuf)) basic_stringbuf;
+template <class _CharT, class _Traits, class _Allocator>
+    class _LIBCPP_PREFERRED_NAME(istringstream) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wistringstream)) basic_istringstream;
+template <class _CharT, class _Traits, class _Allocator>
+    class _LIBCPP_PREFERRED_NAME(ostringstream) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wostringstream)) basic_ostringstream;
+template <class _CharT, class _Traits, class _Allocator>
+    class _LIBCPP_PREFERRED_NAME(stringstream) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wstringstream)) basic_stringstream;
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___FWD_SSTREAM_H
diff --git a/libcxx/include/__fwd/streambuf.h b/libcxx/include/__fwd/streambuf.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__fwd/streambuf.h
@@ -0,0 +1,35 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___FWD_STREAMBUF_H
+#define _LIBCPP___FWD_STREAMBUF_H
+
+#include <__config>
+#include <__fwd/string.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _CharT, class _Traits = char_traits<_CharT> >
+    class _LIBCPP_TEMPLATE_VIS basic_streambuf;
+
+typedef basic_streambuf<char>        streambuf;
+
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+typedef basic_streambuf<wchar_t>     wstreambuf;
+#endif
+
+template <class _CharT, class _Traits>
+    class _LIBCPP_PREFERRED_NAME(streambuf) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wstreambuf)) basic_streambuf;
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___FWD_STREAMBUF_H
diff --git a/libcxx/include/fstream b/libcxx/include/fstream
--- a/libcxx/include/fstream
+++ b/libcxx/include/fstream
@@ -183,6 +183,7 @@
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__availability>
 #include <__config>
+#include <__fwd/fstream.h>
 #include <__locale>
 #include <__utility/move.h>
 #include <__utility/swap.h>
diff --git a/libcxx/include/ios b/libcxx/include/ios
--- a/libcxx/include/ios
+++ b/libcxx/include/ios
@@ -217,6 +217,7 @@
 #endif
 
 #include <__assert> // all public C++ headers provide the assertion handler
+#include <__fwd/ios.h>
 #include <__ios/fpos.h>
 #include <__locale>
 #include <__system_error/error_category.h>
diff --git a/libcxx/include/iosfwd b/libcxx/include/iosfwd
--- a/libcxx/include/iosfwd
+++ b/libcxx/include/iosfwd
@@ -96,6 +96,12 @@
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
+#include <__fwd/fstream.h>
+#include <__fwd/ios.h>
+#include <__fwd/istream.h>
+#include <__fwd/ostream.h>
+#include <__fwd/sstream.h>
+#include <__fwd/streambuf.h>
 #include <__fwd/string.h>
 #include <__mbstate_t.h>
 #include <version>
@@ -108,112 +114,11 @@
 
 class _LIBCPP_TYPE_VIS ios_base;
 
-template <class _CharT, class _Traits = char_traits<_CharT> >
-    class _LIBCPP_TEMPLATE_VIS basic_ios;
-
-template <class _CharT, class _Traits = char_traits<_CharT> >
-    class _LIBCPP_TEMPLATE_VIS basic_streambuf;
-template <class _CharT, class _Traits = char_traits<_CharT> >
-    class _LIBCPP_TEMPLATE_VIS basic_istream;
-template <class _CharT, class _Traits = char_traits<_CharT> >
-    class _LIBCPP_TEMPLATE_VIS basic_ostream;
-template <class _CharT, class _Traits = char_traits<_CharT> >
-    class _LIBCPP_TEMPLATE_VIS basic_iostream;
-
-template <class _CharT, class _Traits = char_traits<_CharT>,
-          class _Allocator = allocator<_CharT> >
-    class _LIBCPP_TEMPLATE_VIS basic_stringbuf;
-template <class _CharT, class _Traits = char_traits<_CharT>,
-          class _Allocator = allocator<_CharT> >
-    class _LIBCPP_TEMPLATE_VIS basic_istringstream;
-template <class _CharT, class _Traits = char_traits<_CharT>,
-          class _Allocator = allocator<_CharT> >
-    class _LIBCPP_TEMPLATE_VIS basic_ostringstream;
-template <class _CharT, class _Traits = char_traits<_CharT>,
-          class _Allocator = allocator<_CharT> >
-    class _LIBCPP_TEMPLATE_VIS basic_stringstream;
-
-template <class _CharT, class _Traits = char_traits<_CharT> >
-    class _LIBCPP_TEMPLATE_VIS basic_filebuf;
-template <class _CharT, class _Traits = char_traits<_CharT> >
-    class _LIBCPP_TEMPLATE_VIS basic_ifstream;
-template <class _CharT, class _Traits = char_traits<_CharT> >
-    class _LIBCPP_TEMPLATE_VIS basic_ofstream;
-template <class _CharT, class _Traits = char_traits<_CharT> >
-    class _LIBCPP_TEMPLATE_VIS basic_fstream;
-
 template <class _CharT, class _Traits = char_traits<_CharT> >
     class _LIBCPP_TEMPLATE_VIS istreambuf_iterator;
 template <class _CharT, class _Traits = char_traits<_CharT> >
     class _LIBCPP_TEMPLATE_VIS ostreambuf_iterator;
 
-typedef basic_ios<char>              ios;
-#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
-typedef basic_ios<wchar_t>           wios;
-#endif
-
-typedef basic_streambuf<char>        streambuf;
-typedef basic_istream<char>          istream;
-typedef basic_ostream<char>          ostream;
-typedef basic_iostream<char>         iostream;
-
-typedef basic_stringbuf<char>        stringbuf;
-typedef basic_istringstream<char>    istringstream;
-typedef basic_ostringstream<char>    ostringstream;
-typedef basic_stringstream<char>     stringstream;
-
-typedef basic_filebuf<char>          filebuf;
-typedef basic_ifstream<char>         ifstream;
-typedef basic_ofstream<char>         ofstream;
-typedef basic_fstream<char>          fstream;
-
-#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
-typedef basic_streambuf<wchar_t>     wstreambuf;
-typedef basic_istream<wchar_t>       wistream;
-typedef basic_ostream<wchar_t>       wostream;
-typedef basic_iostream<wchar_t>      wiostream;
-
-typedef basic_stringbuf<wchar_t>     wstringbuf;
-typedef basic_istringstream<wchar_t> wistringstream;
-typedef basic_ostringstream<wchar_t> wostringstream;
-typedef basic_stringstream<wchar_t>  wstringstream;
-
-typedef basic_filebuf<wchar_t>       wfilebuf;
-typedef basic_ifstream<wchar_t>      wifstream;
-typedef basic_ofstream<wchar_t>      wofstream;
-typedef basic_fstream<wchar_t>       wfstream;
-#endif
-
-template <class _CharT, class _Traits>
-    class _LIBCPP_PREFERRED_NAME(ios) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wios)) basic_ios;
-
-template <class _CharT, class _Traits>
-    class _LIBCPP_PREFERRED_NAME(streambuf) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wstreambuf)) basic_streambuf;
-template <class _CharT, class _Traits>
-    class _LIBCPP_PREFERRED_NAME(istream) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wistream)) basic_istream;
-template <class _CharT, class _Traits>
-    class _LIBCPP_PREFERRED_NAME(ostream) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wostream)) basic_ostream;
-template <class _CharT, class _Traits>
-    class _LIBCPP_PREFERRED_NAME(iostream) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wiostream)) basic_iostream;
-
-template <class _CharT, class _Traits, class _Allocator>
-    class _LIBCPP_PREFERRED_NAME(stringbuf) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wstringbuf)) basic_stringbuf;
-template <class _CharT, class _Traits, class _Allocator>
-    class _LIBCPP_PREFERRED_NAME(istringstream) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wistringstream)) basic_istringstream;
-template <class _CharT, class _Traits, class _Allocator>
-    class _LIBCPP_PREFERRED_NAME(ostringstream) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wostringstream)) basic_ostringstream;
-template <class _CharT, class _Traits, class _Allocator>
-    class _LIBCPP_PREFERRED_NAME(stringstream) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wstringstream)) basic_stringstream;
-
-template <class _CharT, class _Traits>
-    class _LIBCPP_PREFERRED_NAME(filebuf) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wfilebuf)) basic_filebuf;
-template <class _CharT, class _Traits>
-    class _LIBCPP_PREFERRED_NAME(ifstream) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wifstream)) basic_ifstream;
-template <class _CharT, class _Traits>
-    class _LIBCPP_PREFERRED_NAME(ofstream) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wofstream)) basic_ofstream;
-template <class _CharT, class _Traits>
-    class _LIBCPP_PREFERRED_NAME(fstream) _LIBCPP_IF_WIDE_CHARACTERS(_LIBCPP_PREFERRED_NAME(wfstream)) basic_fstream;
-
 template <class _State>             class _LIBCPP_TEMPLATE_VIS fpos;
 typedef fpos<mbstate_t>    streampos;
 typedef fpos<mbstate_t>    wstreampos;
@@ -223,13 +128,6 @@
 typedef fpos<mbstate_t>    u16streampos;
 typedef fpos<mbstate_t>    u32streampos;
 
-#if defined(_NEWLIB_VERSION)
-// On newlib, off_t is 'long int'
-typedef long int streamoff;         // for char_traits in <string>
-#else
-typedef long long streamoff;        // for char_traits in <string>
-#endif
-
 // Include other forward declarations here
 template <class _Tp, class _Alloc = allocator<_Tp> >
 class _LIBCPP_TEMPLATE_VIS vector;
diff --git a/libcxx/include/istream b/libcxx/include/istream
--- a/libcxx/include/istream
+++ b/libcxx/include/istream
@@ -160,6 +160,7 @@
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
+#include <__fwd/istream.h>
 #include <__iterator/istreambuf_iterator.h>
 #include <__type_traits/conjunction.h>
 #include <__type_traits/enable_if.h>
diff --git a/libcxx/include/module.modulemap.in b/libcxx/include/module.modulemap.in
--- a/libcxx/include/module.modulemap.in
+++ b/libcxx/include/module.modulemap.in
@@ -1012,6 +1012,14 @@
   module iosfwd {
     header "iosfwd"
     export *
+    module __iosfwd {
+      module fstream_fwd   { private header "__fwd/fstream.h" }
+      module ios_fwd       { private header "__fwd/ios.h" }
+      module istream_fwd   { private header "__fwd/istream.h" }
+      module ostream_fwd   { private header "__fwd/ostream.h" }
+      module sstream_fwd   { private header "__fwd/sstream.h" }
+      module streambuf_fwd { private header "__fwd/streambuf.h" }
+    }
   }
   module iostream {
     @requires_LIBCXX_ENABLE_LOCALIZATION@
diff --git a/libcxx/include/ostream b/libcxx/include/ostream
--- a/libcxx/include/ostream
+++ b/libcxx/include/ostream
@@ -166,6 +166,7 @@
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
 #include <__exception/operations.h>
+#include <__fwd/ostream.h>
 #include <__memory/shared_ptr.h>
 #include <__memory/unique_ptr.h>
 #include <__system_error/error_code.h>
diff --git a/libcxx/include/sstream b/libcxx/include/sstream
--- a/libcxx/include/sstream
+++ b/libcxx/include/sstream
@@ -187,6 +187,7 @@
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
+#include <__fwd/sstream.h>
 #include <__utility/swap.h>
 #include <istream>
 #include <ostream>
diff --git a/libcxx/include/streambuf b/libcxx/include/streambuf
--- a/libcxx/include/streambuf
+++ b/libcxx/include/streambuf
@@ -109,6 +109,7 @@
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
+#include <__fwd/streambuf.h>
 #include <cstdint>
 #include <ios>
 #include <iosfwd>
diff --git a/libcxx/test/libcxx/private_headers.verify.cpp b/libcxx/test/libcxx/private_headers.verify.cpp
--- a/libcxx/test/libcxx/private_headers.verify.cpp
+++ b/libcxx/test/libcxx/private_headers.verify.cpp
@@ -427,11 +427,17 @@
 #include <__functional/unwrap_ref.h> // expected-error@*:* {{use of private header from outside its module: '__functional/unwrap_ref.h'}}
 #include <__functional/weak_result_type.h> // expected-error@*:* {{use of private header from outside its module: '__functional/weak_result_type.h'}}
 #include <__fwd/array.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/array.h'}}
+#include <__fwd/fstream.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/fstream.h'}}
 #include <__fwd/get.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/get.h'}}
 #include <__fwd/hash.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/hash.h'}}
+#include <__fwd/ios.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/ios.h'}}
+#include <__fwd/istream.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/istream.h'}}
 #include <__fwd/memory_resource.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/memory_resource.h'}}
+#include <__fwd/ostream.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/ostream.h'}}
 #include <__fwd/pair.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/pair.h'}}
 #include <__fwd/span.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/span.h'}}
+#include <__fwd/sstream.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/sstream.h'}}
+#include <__fwd/streambuf.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/streambuf.h'}}
 #include <__fwd/string.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/string.h'}}
 #include <__fwd/string_view.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/string_view.h'}}
 #include <__fwd/subrange.h> // expected-error@*:* {{use of private header from outside its module: '__fwd/subrange.h'}}
diff --git a/libcxx/utils/data/ignore_format.txt b/libcxx/utils/data/ignore_format.txt
--- a/libcxx/utils/data/ignore_format.txt
+++ b/libcxx/utils/data/ignore_format.txt
@@ -398,8 +398,14 @@
 libcxx/include/__functional/unwrap_ref.h
 libcxx/include/__functional/weak_result_type.h
 libcxx/include/future
+libcxx/include/__fwd/fstream.h
 libcxx/include/__fwd/get.h
+libcxx/include/__fwd/ios.h
+libcxx/include/__fwd/istream.h
+libcxx/include/__fwd/ostream.h
 libcxx/include/__fwd/span.h
+libcxx/include/__fwd/sstream.h
+libcxx/include/__fwd/streambuf.h
 libcxx/include/__fwd/string_view.h
 libcxx/include/__fwd/subrange.h
 libcxx/include/__hash_table