[llvm-bugs] [Bug 31178] New: Can't test for __builtin_offsetof

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https://llvm.org/bugs/show_bug.cgi?id=31178

            Bug ID: 31178
           Summary: Can't test for __builtin_offsetof
           Product: clang
           Version: 3.9
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: sitsofe at yahoo.com
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org
    Classification: Unclassified

Description of the problem:
It's not possible to test for __builtin_offsetof using __has_builtin.

Steps to reproduce:
1. Create check_offsetof.c:
#include <stdio.h>
#include <stddef.h>

#ifndef __has_builtin         // Optional of course.
#define __has_builtin(x) 0    // Compatibility with non-clang compilers.
#endif

struct my_struct {
        long long a;
        int b;
};

int main(void) {
#if __has_builtin(__builtin_offsetof)
        printf("Has __builtin_offsetof\n");
#else
        printf("Does NOT have __builtin_offsetof\n");
#endif

        printf("__builtin_offsetof(struct my_struct, b): %lu\n",
                        __builtin_offsetof(struct my_struct, b));
        return 0;
}
2. Compile with
clang check_offsetof.c -o check_offsetof
3. Run
./check_offsetof

Expected result:
Compilation error at 2 or output like
Has __builtin_offsetof
__builtin_offsetof(struct my_struct, b): 8

Actual result:
Output like
Does NOT have __builtin_offsetof
__builtin_offsetof(struct my_struct, b): 8

Additional information:
http://clang.llvm.org/docs/LanguageExtensions.html#has-builtin suggests it
should be possible to use __has_builtin to test for a builtin and this works
for things like __builtin_trap just not __builtin_offsetof.

Version information:
clang version 3.9.0 (branches/release_39 280336)
Target: x86_64-unknown-linux-gnu

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