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.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@260337 91177308-0d34-0410-b5e6-96231b3b80d8

include/string.h

@@ -0,0 +1,110 @@
+// -*- C++ -*-
+//===--------------------------- string.h ---------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_STRING_H
+#define _LIBCPP_STRING_H
+
+/*
+    string.h synopsis
+
+Macros:
+
+    NULL
+
+Types:
+
+    size_t
+
+void* memcpy(void* restrict s1, const void* restrict s2, size_t n);
+void* memmove(void* s1, const void* s2, size_t n);
+char* strcpy (char* restrict s1, const char* restrict s2);
+char* strncpy(char* restrict s1, const char* restrict s2, size_t n);
+char* strcat (char* restrict s1, const char* restrict s2);
+char* strncat(char* restrict s1, const char* restrict s2, size_t n);
+int memcmp(const void* s1, const void* s2, size_t n);
+int strcmp (const char* s1, const char* s2);
+int strncmp(const char* s1, const char* s2, size_t n);
+int strcoll(const char* s1, const char* s2);
+size_t strxfrm(char* restrict s1, const char* restrict s2, size_t n);
+const void* memchr(const void* s, int c, size_t n);
+      void* memchr(      void* s, int c, size_t n);
+const char* strchr(const char* s, int c);
+      char* strchr(      char* s, int c);
+size_t strcspn(const char* s1, const char* s2);
+const char* strpbrk(const char* s1, const char* s2);
+      char* strpbrk(      char* s1, const char* s2);
+const char* strrchr(const char* s, int c);
+      char* strrchr(      char* s, int c);
+size_t strspn(const char* s1, const char* s2);
+const char* strstr(const char* s1, const char* s2);
+      char* strstr(      char* s1, const char* s2);
+char* strtok(char* restrict s1, const char* restrict s2);
+void* memset(void* s, int c, size_t n);
+char* strerror(int errnum);
+size_t strlen(const char* s);
+
+*/
+
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+#include_next <string.h>
+
+// MSVCRT, GNU libc and its derivates already have the correct prototype in
+// <string.h> if __cplusplus is defined. This macro can be defined by users if
+// their C library provides the right signature.
+#if defined(__GLIBC__) || defined(_LIBCPP_MSVCRT) || defined(__sun__) || \
+    defined(_STRING_H_CPLUSPLUS_98_CONFORMANCE_)
+#define _LIBCPP_STRING_H_HAS_CONST_OVERLOADS
+#endif
+
+#if defined(__cplusplus) && !defined(_LIBCPP_STRING_H_HAS_CONST_OVERLOADS) && defined(_LIBCPP_PREFERRED_OVERLOAD)
+extern "C++" {
+inline _LIBCPP_INLINE_VISIBILITY
+char* __libcpp_strchr(const char* __s, int __c) {return (char*)strchr(__s, __c);}
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_PREFERRED_OVERLOAD
+const char* strchr(const char* __s, int __c) {return __libcpp_strchr(__s, __c);}
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_PREFERRED_OVERLOAD
+      char* strchr(      char* __s, int __c) {return __libcpp_strchr(__s, __c);}
+
+inline _LIBCPP_INLINE_VISIBILITY
+char* __libcpp_strpbrk(const char* __s1, const char* __s2) {return (char*)strpbrk(__s1, __s2);}
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_PREFERRED_OVERLOAD
+const char* strpbrk(const char* __s1, const char* __s2) {return __libcpp_strpbrk(__s1, __s2);}
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_PREFERRED_OVERLOAD
+      char* strpbrk(      char* __s1, const char* __s2) {return __libcpp_strpbrk(__s1, __s2);}
+
+inline _LIBCPP_INLINE_VISIBILITY
+char* __libcpp_strrchr(const char* __s, int __c) {return (char*)strrchr(__s, __c);}
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_PREFERRED_OVERLOAD
+const char* strrchr(const char* __s, int __c) {return __libcpp_strrchr(__s, __c);}
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_PREFERRED_OVERLOAD
+      char* strrchr(      char* __s, int __c) {return __libcpp_strrchr(__s, __c);}
+
+inline _LIBCPP_INLINE_VISIBILITY
+void* __libcpp_memchr(const void* __s, int __c, size_t __n) {return (void*)memchr(__s, __c, __n);}
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_PREFERRED_OVERLOAD
+const void* memchr(const void* __s, int __c, size_t __n) {return __libcpp_memchr(__s, __c, __n);}
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_PREFERRED_OVERLOAD
+      void* memchr(      void* __s, int __c, size_t __n) {return __libcpp_memchr(__s, __c, __n);}
+
+inline _LIBCPP_INLINE_VISIBILITY
+char* __libcpp_strstr(const char* __s1, const char* __s2) {return (char*)strstr(__s1, __s2);}
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_PREFERRED_OVERLOAD
+const char* strstr(const char* __s1, const char* __s2) {return __libcpp_strstr(__s1, __s2);}
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_PREFERRED_OVERLOAD
+      char* strstr(      char* __s1, const char* __s2) {return __libcpp_strstr(__s1, __s2);}
+}
+#endif
+
+#endif  // _LIBCPP_STRING_H