Fix overload sets of strchr, strpbrk, strrchr, memchr and strstr from

<string.h> and wcschr, wcspbrk, wcsrchr, wmemchr, and wcsstr from <wchar.h> to
provide a const-correct overload set even when the underlying C library does
not.

This change adds a new macro, _LIBCPP_PREFERRED_OVERLOAD, which (if defined)
specifies that a given overload is a better match than an otherwise equally
good function declaration without the overload. This is implemented in modern
versions of Clang via __attribute__((enable_if)), and not elsewhere.

We use this new macro to define overloads in the global namespace for these
functions that displace the overloads provided by the C library, unless we
believe the C library is already providing the correct signatures.

llvm-svn: 260337

2 files changed, 22 insertions, 10 deletions

diff --git a/libcxx/test/std/strings/c.strings/cstring.pass.cpp b/libcxx/test/std/strings/c.strings/cstring.pass.cpp
index 20f4050cfbd..63f86d35061 100644
--- a/libcxx/test/std/strings/c.strings/cstring.pass.cpp
+++ b/libcxx/test/std/strings/c.strings/cstring.pass.cpp
@@ -34,17 +34,12 @@ int main()
     static_assert((std::is_same<decltype(std::strncmp(cpc, cpc, s)), int>::value), "");
     static_assert((std::is_same<decltype(std::strcoll(cpc, cpc)), int>::value), "");
     static_assert((std::is_same<decltype(std::strxfrm(cp, cpc, s)), std::size_t>::value), "");
-//    static_assert((std::is_same<decltype(std::memchr(vpc, 0, s)), const void*>::value), "");
     static_assert((std::is_same<decltype(std::memchr(vp, 0, s)), void*>::value), "");
-//    static_assert((std::is_same<decltype(std::strchr(cpc, 0)), const char*>::value), "");
     static_assert((std::is_same<decltype(std::strchr(cp, 0)), char*>::value), "");
     static_assert((std::is_same<decltype(std::strcspn(cpc, cpc)), std::size_t>::value), "");
-//    static_assert((std::is_same<decltype(std::strpbrk(cpc, cpc)), const char*>::value), "");
     static_assert((std::is_same<decltype(std::strpbrk(cp, cpc)), char*>::value), "");
-//    static_assert((std::is_same<decltype(std::strrchr(cpc, 0)), const char*>::value), "");
     static_assert((std::is_same<decltype(std::strrchr(cp, 0)), char*>::value), "");
     static_assert((std::is_same<decltype(std::strspn(cpc, cpc)), std::size_t>::value), "");
-//    static_assert((std::is_same<decltype(std::strstr(cpc, cpc)), const char*>::value), "");
     static_assert((std::is_same<decltype(std::strstr(cp, cpc)), char*>::value), "");
 #ifndef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
     static_assert((std::is_same<decltype(std::strtok(cp, cpc)), char*>::value), "");
@@ -52,4 +47,15 @@ int main()
     static_assert((std::is_same<decltype(std::memset(vp, 0, s)), void*>::value), "");
     static_assert((std::is_same<decltype(std::strerror(0)), char*>::value), "");
     static_assert((std::is_same<decltype(std::strlen(cpc)), std::size_t>::value), "");
+
+    // These tests fail on systems whose C library doesn't provide a correct overload
+    // set for strchr, strpbrk, strrchr, strstr, and memchr, unless the compiler is
+    // a suitably recent version of Clang.
+#if !defined(__APPLE__) || defined(_LIBCPP_PREFERRED_OVERLOAD)
+    static_assert((std::is_same<decltype(std::memchr(vpc, 0, s)), const void*>::value), "");
+    static_assert((std::is_same<decltype(std::strchr(cpc, 0)), const char*>::value), "");
+    static_assert((std::is_same<decltype(std::strpbrk(cpc, cpc)), const char*>::value), "");
+    static_assert((std::is_same<decltype(std::strrchr(cpc, 0)), const char*>::value), "");
+    static_assert((std::is_same<decltype(std::strstr(cpc, cpc)), const char*>::value), "");
+#endif
 }
diff --git a/libcxx/test/std/strings/c.strings/cwchar.pass.cpp b/libcxx/test/std/strings/c.strings/cwchar.pass.cpp
index 93d2bb1efa5..85b861017fd 100644
--- a/libcxx/test/std/strings/c.strings/cwchar.pass.cpp
+++ b/libcxx/test/std/strings/c.strings/cwchar.pass.cpp
@@ -73,19 +73,14 @@ int main()
     static_assert((std::is_same<decltype(std::wcscoll(L"", L"")), int>::value), "");
     static_assert((std::is_same<decltype(std::wcsncmp(L"", L"", s)), int>::value), "");
     static_assert((std::is_same<decltype(std::wcsxfrm(ws, L"", s)), std::size_t>::value), "");
-    static_assert((std::is_same<decltype(std::wcschr((const wchar_t*)0, L' ')), const wchar_t*>::value), "");
     static_assert((std::is_same<decltype(std::wcschr((wchar_t*)0, L' ')), wchar_t*>::value), "");
     static_assert((std::is_same<decltype(std::wcscspn(L"", L"")), std::size_t>::value), "");
     static_assert((std::is_same<decltype(std::wcslen(L"")), std::size_t>::value), "");
-    static_assert((std::is_same<decltype(std::wcspbrk((const wchar_t*)0, L"")), const wchar_t*>::value), "");
     static_assert((std::is_same<decltype(std::wcspbrk((wchar_t*)0, L"")), wchar_t*>::value), "");
-    static_assert((std::is_same<decltype(std::wcsrchr((const wchar_t*)0, L' ')), const wchar_t*>::value), "");
     static_assert((std::is_same<decltype(std::wcsrchr((wchar_t*)0, L' ')), wchar_t*>::value), "");
     static_assert((std::is_same<decltype(std::wcsspn(L"", L"")), std::size_t>::value), "");
-    static_assert((std::is_same<decltype(std::wcsstr((const wchar_t*)0, L"")), const wchar_t*>::value), "");
     static_assert((std::is_same<decltype(std::wcsstr((wchar_t*)0, L"")), wchar_t*>::value), "");
     static_assert((std::is_same<decltype(std::wcstok(ws, L"", (wchar_t**)0)), wchar_t*>::value), "");
-    static_assert((std::is_same<decltype(std::wmemchr((const wchar_t*)0, L' ', s)), const wchar_t*>::value), "");
     static_assert((std::is_same<decltype(std::wmemchr((wchar_t*)0, L' ', s)), wchar_t*>::value), "");
     static_assert((std::is_same<decltype(std::wmemcmp(L"", L"", s)), int>::value), "");
     static_assert((std::is_same<decltype(std::wmemcpy(ws, L"", s)), wchar_t*>::value), "");
@@ -101,6 +96,17 @@ int main()
     static_assert((std::is_same<decltype(std::mbsrtowcs(ws, (const char**)0, s, &mb)), std::size_t>::value), "");
     static_assert((std::is_same<decltype(std::wcsrtombs(ns, (const wchar_t**)0, s, &mb)), std::size_t>::value), "");
 
+    // These tests fail on systems whose C library doesn't provide a correct overload
+    // set for wcschr, wcspbrk, wcsrchr, wcsstr, and wmemchr, unless the compiler is
+    // a suitably recent version of Clang.
+#if !defined(__APPLE__) || defined(_LIBCPP_PREFERRED_OVERLOAD)
+    static_assert((std::is_same<decltype(std::wcschr((const wchar_t*)0, L' ')), const wchar_t*>::value), "");
+    static_assert((std::is_same<decltype(std::wcspbrk((const wchar_t*)0, L"")), const wchar_t*>::value), "");
+    static_assert((std::is_same<decltype(std::wcsrchr((const wchar_t*)0, L' ')), const wchar_t*>::value), "");
+    static_assert((std::is_same<decltype(std::wcsstr((const wchar_t*)0, L"")), const wchar_t*>::value), "");
+    static_assert((std::is_same<decltype(std::wmemchr((const wchar_t*)0, L' ', s)), const wchar_t*>::value), "");
+#endif
+
 #ifndef _LIBCPP_HAS_NO_STDIN
     static_assert((std::is_same<decltype(std::getwchar()), std::wint_t>::value), "");
     static_assert((std::is_same<decltype(std::vwscanf(L"", va)), int>::value), "");