[libc++] Consistently enable __CORRECT_ISO_CPP_WCHAR_H_PROTO in mbstate.

In libc++'s `wchar.h`, before we forward to the system `wchar.h`, we set `__CORRECT_ISO_CPP_WCHAR_H_PROTO` to ensure it defines the correct signature (e.g. `extern "C++" const wchar_t *wmemchr` and not `extern wchar_t *wmemchr`). After D148542, there are cases where we include the system `wchar.h` from within `__mbstate_t.h` without setting that, and so we get a function type mismatch if we transitively include `wchar.h` through multiple headers in a modules-enabled build. Consistently setting it here resolves those build errors.

Alternative 1: we could put this in `__config` instead. I chose to put it here for a more limited scope.

Alternative 2: we could patch `wchar.h` itself to work correctly and remove references `__CORRECT_ISO_CPP_WCHAR_H_PROTO` from libc++ entirely. It does already set it, but with an additional condition that it is being built by GCC >= 4.4. Clang does pretend to be GCC via `__GNUC__` etc. which can be controlled via `-fgnuc-version` command line flags, but that might have other consequences.

Reviewed By: ldionne, #libc, MaskRay

Differential Revision: https://reviews.llvm.org/D150015

4 files changed, 60 insertions, 0 deletions

diff --git a/libcxx/include/__mbstate_t.h b/libcxx/include/__mbstate_t.h
index d793787fa0cd..000af71119f4 100644
--- 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
index db624cea2bee..eeccf3e937ff 100644
--- a/libcxx/include/wchar.h
+++ b/libcxx/include/wchar.h
@@ -116,6 +116,8 @@ size_t wcsrtombs(char* restrict dst, const wchar_t** restrict src, size_t len,
 #  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
index 000000000000..4b5460a54859
--- /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
index 000000000000..0222ac018d68
--- /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}}
+}