diff --git a/libcxx/include/__mbstate_t.h b/libcxx/include/__mbstate_t.h
index c23ea7113ca70..1e1cf7b27c799 100644
--- a/libcxx/include/__mbstate_t.h
+++ b/libcxx/include/__mbstate_t.h
@@ -43,8 +43,15 @@
 #  include <bits/types/mbstate_t.h> // works on most Unixes
 #elif __has_include(<sys/_types/_mbstate_t.h>)
 #  include <sys/_types/_mbstate_t.h> // works on Darwin
+// include_next works differently for module builders.
 #elif __has_include_next(<wchar.h>)
+#  define _LIBCPP_INCLUDE_NEXT_WCHAR
 #  include_next <wchar.h> // use the C standard provider of mbstate_t if present
+#  undef _LIBCPP_INCLUDE_NEXT_WCHAR
+#  ifdef _LIBCPP_WCHAR_NOT_FOUND
+#    undef _LIBCPP_WCHAR_NOT_FOUND
+#    include <uchar.h>
+#  endif
 #elif __has_include_next(<uchar.h>)
 #  include_next <uchar.h> // Try <uchar.h> in absence of <wchar.h> for mbstate_t
 #else
diff --git a/libcxx/include/__wchar.h b/libcxx/include/__wchar.h
new file mode 100644
index 0000000000000..a5ebc32da7ebb
--- /dev/null
+++ b/libcxx/include/__wchar.h
@@ -0,0 +1,105 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// Lack of header guards is intentional.
+// This file should only ever be included by wchar.h, which provides its own
+// header guard. This prevents macro hiding issues with modules where cwchar
+// complains that _LIBCPP_WCHAR_H isn't present.
+
+#include <__config>
+#include <__mbstate_t.h> // provide mbstate_t
+#include <stddef.h>      // provide size_t
+
+// include_next doesn't work with modules.
+#if __has_include_next(<wchar.h>)
+#  define _LIBCPP_INCLUDE_NEXT_WCHAR
+#  include_next <wchar.h>
+#  undef _LIBCPP_INCLUDE_NEXT_WCHAR
+#endif
+
+// Determine whether we have const-correct overloads for wcschr and friends.
+#if defined(_WCHAR_H_CPLUSPLUS_98_CONFORMANCE_)
+#  define _LIBCPP_WCHAR_H_HAS_CONST_OVERLOADS 1
+#elif defined(__GLIBC_PREREQ)
+#  if __GLIBC_PREREQ(2, 10)
+#    define _LIBCPP_WCHAR_H_HAS_CONST_OVERLOADS 1
+#  endif
+#elif defined(_LIBCPP_MSVCRT)
+#  if defined(_CRT_CONST_CORRECT_OVERLOADS)
+#    define _LIBCPP_WCHAR_H_HAS_CONST_OVERLOADS 1
+#  endif
+#endif
+
+#if _LIBCPP_HAS_WIDE_CHARACTERS
+#  if defined(__cplusplus) && !defined(_LIBCPP_WCHAR_H_HAS_CONST_OVERLOADS) && defined(_LIBCPP_PREFERRED_OVERLOAD)
+extern "C++" {
+inline _LIBCPP_HIDE_FROM_ABI wchar_t* __libcpp_wcschr(const wchar_t* __s, wchar_t __c) {
+  return (wchar_t*)wcschr(__s, __c);
+}
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_PREFERRED_OVERLOAD const wchar_t* wcschr(const wchar_t* __s, wchar_t __c) {
+  return __libcpp_wcschr(__s, __c);
+}
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_PREFERRED_OVERLOAD wchar_t* wcschr(wchar_t* __s, wchar_t __c) {
+  return __libcpp_wcschr(__s, __c);
+}
+
+inline _LIBCPP_HIDE_FROM_ABI wchar_t* __libcpp_wcspbrk(const wchar_t* __s1, const wchar_t* __s2) {
+  return (wchar_t*)wcspbrk(__s1, __s2);
+}
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_PREFERRED_OVERLOAD const wchar_t*
+wcspbrk(const wchar_t* __s1, const wchar_t* __s2) {
+  return __libcpp_wcspbrk(__s1, __s2);
+}
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_PREFERRED_OVERLOAD wchar_t* wcspbrk(wchar_t* __s1, const wchar_t* __s2) {
+  return __libcpp_wcspbrk(__s1, __s2);
+}
+
+inline _LIBCPP_HIDE_FROM_ABI wchar_t* __libcpp_wcsrchr(const wchar_t* __s, wchar_t __c) {
+  return (wchar_t*)wcsrchr(__s, __c);
+}
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_PREFERRED_OVERLOAD const wchar_t* wcsrchr(const wchar_t* __s, wchar_t __c) {
+  return __libcpp_wcsrchr(__s, __c);
+}
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_PREFERRED_OVERLOAD wchar_t* wcsrchr(wchar_t* __s, wchar_t __c) {
+  return __libcpp_wcsrchr(__s, __c);
+}
+
+inline _LIBCPP_HIDE_FROM_ABI wchar_t* __libcpp_wcsstr(const wchar_t* __s1, const wchar_t* __s2) {
+  return (wchar_t*)wcsstr(__s1, __s2);
+}
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_PREFERRED_OVERLOAD const wchar_t*
+wcsstr(const wchar_t* __s1, const wchar_t* __s2) {
+  return __libcpp_wcsstr(__s1, __s2);
+}
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_PREFERRED_OVERLOAD wchar_t* wcsstr(wchar_t* __s1, const wchar_t* __s2) {
+  return __libcpp_wcsstr(__s1, __s2);
+}
+
+inline _LIBCPP_HIDE_FROM_ABI wchar_t* __libcpp_wmemchr(const wchar_t* __s, wchar_t __c, size_t __n) {
+  return (wchar_t*)wmemchr(__s, __c, __n);
+}
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_PREFERRED_OVERLOAD const wchar_t*
+wmemchr(const wchar_t* __s, wchar_t __c, size_t __n) {
+  return __libcpp_wmemchr(__s, __c, __n);
+}
+inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_PREFERRED_OVERLOAD wchar_t* wmemchr(wchar_t* __s, wchar_t __c, size_t __n) {
+  return __libcpp_wmemchr(__s, __c, __n);
+}
+}
+#  endif
+
+#  if defined(__cplusplus) && (defined(_LIBCPP_MSVCRT_LIKE) || defined(__MVS__))
+extern "C" {
+size_t mbsnrtowcs(
+    wchar_t* __restrict __dst, const char** __restrict __src, size_t __nmc, size_t __len, mbstate_t* __restrict __ps);
+size_t wcsnrtombs(
+    char* __restrict __dst, const wchar_t** __restrict __src, size_t __nwc, size_t __len, mbstate_t* __restrict __ps);
+} // extern "C"
+#  endif // __cplusplus && (_LIBCPP_MSVCRT || __MVS__)
+#endif   // _LIBCPP_HAS_WIDE_CHARACTERS
diff --git a/libcxx/include/module.modulemap.in b/libcxx/include/module.modulemap.in
index f878e15d70b1a..bc7dc8d3efa10 100644
--- a/libcxx/include/module.modulemap.in
+++ b/libcxx/include/module.modulemap.in
@@ -2420,6 +2420,9 @@ module std_uchar_h [system] {
 module std_wchar_h [system] {
   // <wchar.h> supports being included multiple times with different pre-defined macros
   textual header "wchar.h"
+  // Parts of wchar.h contain function definitions, so cannot be included
+  // multiple times.
+  header "__wchar.h"
 }
 module std_wctype_h [system] {
   header "wctype.h"
diff --git a/libcxx/include/wchar.h b/libcxx/include/wchar.h
index a932dd266b862..633430093df5a 100644
--- a/libcxx/include/wchar.h
+++ b/libcxx/include/wchar.h
@@ -94,7 +94,17 @@ size_t wcsrtombs(char* restrict dst, const wchar_t** restrict src, size_t len,
 
 */
 
-#if defined(__cplusplus) && __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)
+// #include_next has different semantics for module builds.
+// include_next<wchar.h> only works if the current filename is wchar.h,
+// otherwise it just does a regular #include.
+// To solve this, if _LIBCPP_INCLUDE_NEXT_WCHAR is defined, fake an include_next.
+#ifdef _LIBCPP_INCLUDE_NEXT_WCHAR
+#  if __has_include_next(<wchar.h>)
+#    include_next <wchar.h>
+#  elif defined(_LIBCPP_INCLUDE_NEXT_WCHAR)
+#    define _LIBCPP_WCHAR_NOT_FOUND
+#  endif
+#elif defined(__cplusplus) && __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)
 #  include <__cxx03/wchar.h>
 #else
 #  include <__config>
@@ -116,92 +126,12 @@ size_t wcsrtombs(char* restrict dst, const wchar_t** restrict src, size_t len,
 #    include_next <wchar.h>
 #  endif
 
-#  ifndef _LIBCPP_WCHAR_H
+// Place the header guard here to make it visible to cwchar.
+#  if !defined(_LIBCPP_WCHAR_H)
 #    define _LIBCPP_WCHAR_H
-
-#    include <__mbstate_t.h> // provide mbstate_t
-#    include <stddef.h>      // provide size_t
-
-// Determine whether we have const-correct overloads for wcschr and friends.
-#    if defined(_WCHAR_H_CPLUSPLUS_98_CONFORMANCE_)
-#      define _LIBCPP_WCHAR_H_HAS_CONST_OVERLOADS 1
-#    elif defined(__GLIBC_PREREQ)
-#      if __GLIBC_PREREQ(2, 10)
-#        define _LIBCPP_WCHAR_H_HAS_CONST_OVERLOADS 1
-#      endif
-#    elif defined(_LIBCPP_MSVCRT)
-#      if defined(_CRT_CONST_CORRECT_OVERLOADS)
-#        define _LIBCPP_WCHAR_H_HAS_CONST_OVERLOADS 1
-#      endif
-#    endif
-
-#    if _LIBCPP_HAS_WIDE_CHARACTERS
-#      if defined(__cplusplus) && !defined(_LIBCPP_WCHAR_H_HAS_CONST_OVERLOADS) && defined(_LIBCPP_PREFERRED_OVERLOAD)
-extern "C++" {
-inline _LIBCPP_HIDE_FROM_ABI wchar_t* __libcpp_wcschr(const wchar_t* __s, wchar_t __c) {
-  return (wchar_t*)wcschr(__s, __c);
-}
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_PREFERRED_OVERLOAD const wchar_t* wcschr(const wchar_t* __s, wchar_t __c) {
-  return __libcpp_wcschr(__s, __c);
-}
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_PREFERRED_OVERLOAD wchar_t* wcschr(wchar_t* __s, wchar_t __c) {
-  return __libcpp_wcschr(__s, __c);
-}
-
-inline _LIBCPP_HIDE_FROM_ABI wchar_t* __libcpp_wcspbrk(const wchar_t* __s1, const wchar_t* __s2) {
-  return (wchar_t*)wcspbrk(__s1, __s2);
-}
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_PREFERRED_OVERLOAD const wchar_t*
-wcspbrk(const wchar_t* __s1, const wchar_t* __s2) {
-  return __libcpp_wcspbrk(__s1, __s2);
-}
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_PREFERRED_OVERLOAD wchar_t* wcspbrk(wchar_t* __s1, const wchar_t* __s2) {
-  return __libcpp_wcspbrk(__s1, __s2);
-}
-
-inline _LIBCPP_HIDE_FROM_ABI wchar_t* __libcpp_wcsrchr(const wchar_t* __s, wchar_t __c) {
-  return (wchar_t*)wcsrchr(__s, __c);
-}
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_PREFERRED_OVERLOAD const wchar_t* wcsrchr(const wchar_t* __s, wchar_t __c) {
-  return __libcpp_wcsrchr(__s, __c);
-}
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_PREFERRED_OVERLOAD wchar_t* wcsrchr(wchar_t* __s, wchar_t __c) {
-  return __libcpp_wcsrchr(__s, __c);
-}
-
-inline _LIBCPP_HIDE_FROM_ABI wchar_t* __libcpp_wcsstr(const wchar_t* __s1, const wchar_t* __s2) {
-  return (wchar_t*)wcsstr(__s1, __s2);
-}
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_PREFERRED_OVERLOAD const wchar_t*
-wcsstr(const wchar_t* __s1, const wchar_t* __s2) {
-  return __libcpp_wcsstr(__s1, __s2);
-}
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_PREFERRED_OVERLOAD wchar_t* wcsstr(wchar_t* __s1, const wchar_t* __s2) {
-  return __libcpp_wcsstr(__s1, __s2);
-}
-
-inline _LIBCPP_HIDE_FROM_ABI wchar_t* __libcpp_wmemchr(const wchar_t* __s, wchar_t __c, size_t __n) {
-  return (wchar_t*)wmemchr(__s, __c, __n);
-}
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_PREFERRED_OVERLOAD const wchar_t*
-wmemchr(const wchar_t* __s, wchar_t __c, size_t __n) {
-  return __libcpp_wmemchr(__s, __c, __n);
-}
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_PREFERRED_OVERLOAD wchar_t* wmemchr(wchar_t* __s, wchar_t __c, size_t __n) {
-  return __libcpp_wmemchr(__s, __c, __n);
-}
-}
-#      endif
-
-#      if defined(__cplusplus) && (defined(_LIBCPP_MSVCRT_LIKE) || defined(__MVS__))
-extern "C" {
-size_t mbsnrtowcs(
-    wchar_t* __restrict __dst, const char** __restrict __src, size_t __nmc, size_t __len, mbstate_t* __restrict __ps);
-size_t wcsnrtombs(
-    char* __restrict __dst, const wchar_t** __restrict __src, size_t __nwc, size_t __len, mbstate_t* __restrict __ps);
-} // extern "C"
-#      endif // __cplusplus && (_LIBCPP_MSVCRT || __MVS__)
-#    endif   // _LIBCPP_HAS_WIDE_CHARACTERS
-#  endif     // _LIBCPP_WCHAR_H
+// This section is not safe to include multiple times, so it goes in a seperate
+// file which is marked as non textual in the modulemap.
+#    include <__wchar.h>
+#  endif
 
 #endif // defined(__cplusplus) && __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)