[libc++] Diagnose when header search paths are set up incorrectly

An issue I often see in codebases compiled for unusual platforms is
that header search paths are specified manually and are subtly wrong.
For example, people will manually add `-isystem <some-toolchain>/usr/include`,
which ends up messing up the layering of header search paths required by
libc++ (because the C Standard Library now appears *before* libc++ in
the search paths). Without this patch, this will end up causing
compilation errors that are pretty inscrutable. This patch aims to
improve the user experience by diagnosing this issue explicitly.

In all cases I can think of, I would expect that a compilation error
occur if these header search paths are not layered properly. This
should only provide an explicit diagnostic instead of failing due
to seemingly unrelated compilation errors.

Differential Revision: https://reviews.llvm.org/D131441

17 files changed, 154 insertions, 2 deletions

diff --git a/libcxx/include/cctype b/libcxx/include/cctype
index 248f8d9..867e7d5 100644
--- a/libcxx/include/cctype
+++ b/libcxx/include/cctype
@@ -36,8 +36,16 @@ int toupper(int c);
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
+
 #include <ctype.h>
 
+#ifndef _LIBCPP_CTYPE_H
+#   error <cctype> tried including <ctype.h> but didn't find libc++'s <ctype.h> header. \
+          This usually means that your header search paths are not configured properly.  \
+          The header search paths should contain the C++ Standard Library headers before \
+          any C Standard Library.
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
diff --git a/libcxx/include/cerrno b/libcxx/include/cerrno
index e9eacd3..937ec23c 100644
--- a/libcxx/include/cerrno
+++ b/libcxx/include/cerrno
@@ -24,8 +24,17 @@ Macros:
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
+
 #include <errno.h>
 
+#ifndef _LIBCPP_ERRNO_H
+#   error <cerrno> tried including <errno.h> but didn't find libc++'s <errno.h> header. \
+          This usually means that your header search paths are not configured properly. \
+          The header search paths should contain the C++ Standard Library headers before \
+          any C Standard Library, and you are probably using compiler flags that make that \
+          not be the case.
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
diff --git a/libcxx/include/cfenv b/libcxx/include/cfenv
index e42b466..16b3761 100644
--- a/libcxx/include/cfenv
+++ b/libcxx/include/cfenv
@@ -54,8 +54,17 @@ int feupdateenv(const fenv_t* envp);
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
+
 #include <fenv.h>
 
+#ifndef _LIBCPP_FENV_H
+#   error <cfenv> tried including <fenv.h> but didn't find libc++'s <fenv.h> header. \
+          This usually means that your header search paths are not configured properly. \
+          The header search paths should contain the C++ Standard Library headers before \
+          any C Standard Library, and you are probably using compiler flags that make that \
+          not be the case.
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
diff --git a/libcxx/include/cfloat b/libcxx/include/cfloat
index 36a7c51..4f991dd 100644
--- a/libcxx/include/cfloat
+++ b/libcxx/include/cfloat
@@ -71,8 +71,17 @@ Macros:
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
+
 #include <float.h>
 
+#ifndef _LIBCPP_FLOAT_H
+#   error <cfloat> tried including <float.h> but didn't find libc++'s <float.h> header. \
+          This usually means that your header search paths are not configured properly. \
+          The header search paths should contain the C++ Standard Library headers before \
+          any C Standard Library, and you are probably using compiler flags that make that \
+          not be the case.
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
diff --git a/libcxx/include/cinttypes b/libcxx/include/cinttypes
index a4cfe96..0930446 100644
--- a/libcxx/include/cinttypes
+++ b/libcxx/include/cinttypes
@@ -237,8 +237,17 @@ uintmax_t wcstoumax(const wchar_t* restrict nptr, wchar_t** restrict endptr, int
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
 #include <cstdint>
+
 #include <inttypes.h>
 
+#ifndef _LIBCPP_INTTYPES_H
+#   error <cinttypes> tried including <inttypes.h> but didn't find libc++'s <inttypes.h> header. \
+          This usually means that your header search paths are not configured properly. \
+          The header search paths should contain the C++ Standard Library headers before \
+          any C Standard Library, and you are probably using compiler flags that make that \
+          not be the case.
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
diff --git a/libcxx/include/climits b/libcxx/include/climits
index 16800a6..d773af5 100644
--- a/libcxx/include/climits
+++ b/libcxx/include/climits
@@ -39,8 +39,17 @@ Macros:
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
+
 #include <limits.h>
 
+#ifndef _LIBCPP_LIMITS_H
+#   error <climits> tried including <limits.h> but didn't find libc++'s <limits.h> header. \
+          This usually means that your header search paths are not configured properly. \
+          The header search paths should contain the C++ Standard Library headers before \
+          any C Standard Library, and you are probably using compiler flags that make that \
+          not be the case.
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
diff --git a/libcxx/include/clocale b/libcxx/include/clocale
index 8926042..e2ace35 100644
--- a/libcxx/include/clocale
+++ b/libcxx/include/clocale
@@ -36,8 +36,17 @@ lconv* localeconv();
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
+
 #include <locale.h>
 
+#ifndef _LIBCPP_LOCALE_H
+#   error <clocale> tried including <locale.h> but didn't find libc++'s <locale.h> header. \
+          This usually means that your header search paths are not configured properly. \
+          The header search paths should contain the C++ Standard Library headers before \
+          any C Standard Library, and you are probably using compiler flags that make that \
+          not be the case.
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
diff --git a/libcxx/include/cmath b/libcxx/include/cmath
index 65a8531..ef3a678 100644
--- a/libcxx/include/cmath
+++ b/libcxx/include/cmath
@@ -306,10 +306,19 @@ constexpr long double lerp(long double a, long double b, long double t) noexcept
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
-#include <math.h>
 #include <type_traits>
 #include <version>
 
+#include <math.h>
+
+#ifndef _LIBCPP_MATH_H
+#   error <cmath> tried including <math.h> but didn't find libc++'s <math.h> header. \
+          This usually means that your header search paths are not configured properly. \
+          The header search paths should contain the C++ Standard Library headers before \
+          any C Standard Library, and you are probably using compiler flags that make that \
+          not be the case.
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
diff --git a/libcxx/include/csetjmp b/libcxx/include/csetjmp
index 4f40bcb..4c64e83 100644
--- a/libcxx/include/csetjmp
+++ b/libcxx/include/csetjmp
@@ -32,8 +32,17 @@ void longjmp(jmp_buf env, int val);
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
+
 #include <setjmp.h>
 
+#ifndef _LIBCPP_SETJMP_H
+#   error <csetjmp> tried including <setjmp.h> but didn't find libc++'s <setjmp.h> header. \
+          This usually means that your header search paths are not configured properly. \
+          The header search paths should contain the C++ Standard Library headers before \
+          any C Standard Library, and you are probably using compiler flags that make that \
+          not be the case.
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
diff --git a/libcxx/include/cstddef b/libcxx/include/cstddef
index 7be4348..e3c0669 100644
--- a/libcxx/include/cstddef
+++ b/libcxx/include/cstddef
@@ -38,9 +38,18 @@ Types:
 #include <__type_traits/enable_if.h>
 #include <__type_traits/integral_constant.h>
 #include <__type_traits/is_integral.h>
-#include <stddef.h>
 #include <version>
 
+#include <stddef.h>
+
+#ifndef _LIBCPP_STDDEF_H
+#   error <cstddef> tried including <stddef.h> but didn't find libc++'s <stddef.h> header. \
+          This usually means that your header search paths are not configured properly. \
+          The header search paths should contain the C++ Standard Library headers before \
+          any C Standard Library, and you are probably using compiler flags that make that \
+          not be the case.
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
diff --git a/libcxx/include/cstdint b/libcxx/include/cstdint
index 83cda94..829d939 100644
--- a/libcxx/include/cstdint
+++ b/libcxx/include/cstdint
@@ -142,8 +142,17 @@ Types:
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
+
 #include <stdint.h>
 
+#ifndef _LIBCPP_STDINT_H
+#   error <cstdint> tried including <stdint.h> but didn't find libc++'s <stdint.h> header. \
+          This usually means that your header search paths are not configured properly. \
+          The header search paths should contain the C++ Standard Library headers before \
+          any C Standard Library, and you are probably using compiler flags that make that \
+          not be the case.
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
diff --git a/libcxx/include/cstdio b/libcxx/include/cstdio
index d191086..221b0b3 100644
--- a/libcxx/include/cstdio
+++ b/libcxx/include/cstdio
@@ -97,8 +97,17 @@ void perror(const char* s);
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
+
 #include <stdio.h>
 
+#ifndef _LIBCPP_STDIO_H
+#   error <cstdio> tried including <stdio.h> but didn't find libc++'s <stdio.h> header. \
+          This usually means that your header search paths are not configured properly. \
+          The header search paths should contain the C++ Standard Library headers before \
+          any C Standard Library, and you are probably using compiler flags that make that \
+          not be the case.
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
diff --git a/libcxx/include/cstdlib b/libcxx/include/cstdlib
index 457c31f..25c9de5 100644
--- a/libcxx/include/cstdlib
+++ b/libcxx/include/cstdlib
@@ -83,8 +83,17 @@ void *aligned_alloc(size_t alignment, size_t size);                       // C11
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
+
 #include <stdlib.h>
 
+#ifndef _LIBCPP_STDLIB_H
+#   error <cstdlib> tried including <stdlib.h> but didn't find libc++'s <stdlib.h> header. \
+          This usually means that your header search paths are not configured properly. \
+          The header search paths should contain the C++ Standard Library headers before \
+          any C Standard Library, and you are probably using compiler flags that make that \
+          not be the case.
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
diff --git a/libcxx/include/cstring b/libcxx/include/cstring
index 37c92e1..42bbe73 100644
--- a/libcxx/include/cstring
+++ b/libcxx/include/cstring
@@ -58,8 +58,17 @@ size_t strlen(const char* s);
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
+
 #include <string.h>
 
+#ifndef _LIBCPP_STRING_H
+#   error <cstring> tried including <string.h> but didn't find libc++'s <string.h> header. \
+          This usually means that your header search paths are not configured properly. \
+          The header search paths should contain the C++ Standard Library headers before \
+          any C Standard Library, and you are probably using compiler flags that make that \
+          not be the case.
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
diff --git a/libcxx/include/cuchar b/libcxx/include/cuchar
index a4ed585..b685f47 100644
--- a/libcxx/include/cuchar
+++ b/libcxx/include/cuchar
@@ -36,8 +36,17 @@ size_t c32rtomb(char* s, char32_t c32, mbstate_t* ps);
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
+
 #include <uchar.h>
 
+#ifndef _LIBCPP_UCHAR_H
+#   error <cuchar> tried including <uchar.h> but didn't find libc++'s <uchar.h> header. \
+          This usually means that your header search paths are not configured properly. \
+          The header search paths should contain the C++ Standard Library headers before \
+          any C Standard Library, and you are probably using compiler flags that make that \
+          not be the case.
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
diff --git a/libcxx/include/cwchar b/libcxx/include/cwchar
index 5c69ab2..220c817 100644
--- a/libcxx/include/cwchar
+++ b/libcxx/include/cwchar
@@ -105,8 +105,17 @@ size_t wcsrtombs(char* restrict dst, const wchar_t** restrict src, size_t len,
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
 #include <cwctype>
+
 #include <wchar.h>
 
+#ifndef _LIBCPP_WCHAR_H
+#   error <cwchar> tried including <wchar.h> but didn't find libc++'s <wchar.h> header. \
+          This usually means that your header search paths are not configured properly. \
+          The header search paths should contain the C++ Standard Library headers before \
+          any C Standard Library, and you are probably using compiler flags that make that \
+          not be the case.
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
diff --git a/libcxx/include/cwctype b/libcxx/include/cwctype
index 429b00c2..5a2d242 100644
--- a/libcxx/include/cwctype
+++ b/libcxx/include/cwctype
@@ -52,8 +52,17 @@ wctrans_t wctrans(const char* property);
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
 #include <cctype>
+
 #include <wctype.h>
 
+#ifndef _LIBCPP_WCTYPE_H
+#   error <cwctype> tried including <wctype.h> but didn't find libc++'s <wctype.h> header. \
+          This usually means that your header search paths are not configured properly. \
+          The header search paths should contain the C++ Standard Library headers before \
+          any C Standard Library, and you are probably using compiler flags that make that \
+          not be the case.
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif