Do not undefine __has_feature (may break c++ headers)

If the order of <common/defines.h> and c++ header will be wrong the
compilation will be broken.

v2: rename __ch_has_feature to ch_has_feature to fix -Wreserved-id-macro
v3: do not fallback to 0

base/common/defines.h

@@ -2,10 +2,17 @@
 
 /// __has_feature supported only by clang.
 ///
-/// But libcxx/libcxxabi overrides it to 0, thus the checks for __has_feature will be wrong,
-/// undefine it again to avoid such issues.
-#if defined(__has_feature) && !defined(__clang__)
-#  undef __has_feature
+/// But libcxx/libcxxabi overrides it to 0,
+/// thus the checks for __has_feature will be wrong.
+///
+/// NOTE:
+/// - __has_feature cannot be simply undefined,
+///   since this will be broken if some C++ header will be included after
+///   including <common/defines.h>
+/// - it should not have fallback to 0,
+///   since this may create false-positive detection (common problem)
+#if defined(__clang__) && defined(__has_feature)
+#    define ch_has_feature __has_feature
 #endif
 
 #if defined(_MSC_VER)
@@ -40,8 +47,8 @@
 
 /// Check for presence of address sanitizer
 #if !defined(ADDRESS_SANITIZER)
-#    if defined(__has_feature)
-#        if __has_feature(address_sanitizer)
+#    if defined(ch_has_feature)
+#        if ch_has_feature(address_sanitizer)
 #            define ADDRESS_SANITIZER 1
 #        endif
 #    elif defined(__SANITIZE_ADDRESS__)
@@ -50,8 +57,8 @@
 #endif
 
 #if !defined(THREAD_SANITIZER)
-#    if defined(__has_feature)
-#        if __has_feature(thread_sanitizer)
+#    if defined(ch_has_feature)
+#        if ch_has_feature(thread_sanitizer)
 #            define THREAD_SANITIZER 1
 #        endif
 #    elif defined(__SANITIZE_THREAD__)
@@ -60,8 +67,8 @@
 #endif
 
 #if !defined(MEMORY_SANITIZER)
-#    if defined(__has_feature)
-#        if __has_feature(memory_sanitizer)
+#    if defined(ch_has_feature)
+#        if ch_has_feature(memory_sanitizer)
 #            define MEMORY_SANITIZER 1
 #        endif
 #    elif defined(__MEMORY_SANITIZER__)

base/common/phdr_cache.cpp

@@ -15,11 +15,11 @@
 #endif
 
 #define __msan_unpoison(X, Y) // NOLINT
-#if defined(__has_feature)
-#   if __has_feature(memory_sanitizer)
-#       undef __msan_unpoison
-#       include <sanitizer/msan_interface.h>
-#   endif
+#if defined(ch_has_feature)
+#    if ch_has_feature(memory_sanitizer)
+#        undef __msan_unpoison
+#        include <sanitizer/msan_interface.h>
+#    endif
 #endif
 
 #include <link.h>

src/Common/MemorySanitizer.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <common/defines.h>
+
 #ifdef __clang__
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wreserved-id-macro"
@@ -9,14 +11,15 @@
 #define __msan_test_shadow(X, Y) (false)
 #define __msan_print_shadow(X, Y)
 #define __msan_unpoison_string(X)
-#if defined(__has_feature)
-#   if __has_feature(memory_sanitizer)
-#       undef __msan_unpoison
-#       undef __msan_test_shadow
-#       undef __msan_print_shadow
-#       undef __msan_unpoison_string
-#       include <sanitizer/msan_interface.h>
-#   endif
+
+#if defined(ch_has_feature)
+#    if ch_has_feature(memory_sanitizer)
+#        undef __msan_unpoison
+#        undef __msan_test_shadow
+#        undef __msan_print_shadow
+#        undef __msan_unpoison_string
+#        include <sanitizer/msan_interface.h>
+#    endif
 #endif
 
 #ifdef __clang__

src/Common/SymbolIndex.cpp

@@ -60,11 +60,11 @@ Otherwise you will get only exported symbols from program headers.
 #endif
 
 #define __msan_unpoison_string(X) // NOLINT
-#if defined(__has_feature)
-#   if __has_feature(memory_sanitizer)
-#       undef __msan_unpoison_string
-#       include <sanitizer/msan_interface.h>
-#   endif
+#if defined(ch_has_feature)
+#    if ch_has_feature(memory_sanitizer)
+#        undef __msan_unpoison_string
+#        include <sanitizer/msan_interface.h>
+#    endif
 #endif