diff --git a/libcxx/include/__mbstate_t.h b/libcxx/include/__mbstate_t.h
--- a/libcxx/include/__mbstate_t.h
+++ b/libcxx/include/__mbstate_t.h
@@ -27,6 +27,14 @@
 // This does not define std::mbstate_t -- this only brings in the declaration
 // in the global namespace.
 
+// We define this here to support older versions of glibc <wchar.h> that do
+// not define this for clang. This is also set in libc++'s <wchar.h> header,
+// and we need to do so here too to avoid a different function signature given
+// a different include order.
+#ifdef __cplusplus
+#  define __CORRECT_ISO_CPP_WCHAR_H_PROTO
+#endif
+
 #if __has_include(<bits/types/mbstate_t.h>)
 #   include <bits/types/mbstate_t.h> // works on most Unixes
 #elif __has_include(<sys/_types/_mbstate_t.h>)
diff --git a/libcxx/include/wchar.h b/libcxx/include/wchar.h
--- a/libcxx/include/wchar.h
+++ b/libcxx/include/wchar.h
@@ -116,6 +116,8 @@
 #  pragma GCC system_header
 #endif
 
+// We define this here to support older versions of glibc <wchar.h> that do
+// not define this for clang.
 #ifdef __cplusplus
 #define __CORRECT_ISO_CPP_WCHAR_H_PROTO
 #endif
diff --git a/libcxx/test/std/strings/c.strings/cwchar_include_order1.compile.verify.cpp b/libcxx/test/std/strings/c.strings/cwchar_include_order1.compile.verify.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/strings/c.strings/cwchar_include_order1.compile.verify.cpp
@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <cwchar>
+
+// XFAIL: no-wide-characters
+
+// Tests that include ordering does not affect the definition of wcsstr.
+// See: https://llvm.org/PR62638
+
+// clang-format off
+#include <cwchar>
+#include <iosfwd>
+// clang-format on
+
+void func() {
+  wchar_t* v1;
+  const wchar_t* cv2 = L"/";
+  v1 = wcsstr(cv2, L"/"); // expected-error {{assigning to 'wchar_t *' from 'const wchar_t *' discards qualifiers}}
+}
diff --git a/libcxx/test/std/strings/c.strings/cwchar_include_order2.compile.verify.cpp b/libcxx/test/std/strings/c.strings/cwchar_include_order2.compile.verify.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/strings/c.strings/cwchar_include_order2.compile.verify.cpp
@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <cwchar>
+
+// XFAIL: no-wide-characters
+
+// Tests that include ordering does not affect the definition of wcsstr.
+// See: https://llvm.org/PR62638
+
+// clang-format off
+#include <iosfwd>
+#include <cwchar>
+// clang-format on
+
+void func() {
+  wchar_t* v1;
+  const wchar_t* cv2 = L"/";
+  v1 = wcsstr(cv2, L"/"); // expected-error {{assigning to 'wchar_t *' from 'const wchar_t *' discards qualifiers}}
+}