[LLVMbugs] [Bug 20667] New: va_arg error on a trivially copyable class

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Thu Aug 14 16:25:13 PDT 2014


            Bug ID: 20667
           Summary: va_arg error on a trivially copyable class
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++11
          Assignee: unassignedclangbugs at nondot.org
          Reporter: msebor at gmail.com
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Based on my reading of paragraph 7 of 5.2.2 of C++ 11, an object of a type with
a trivial copy and move constructors and destructor can be passed as an
argument to a function declared with the ellipsis that has no corresponding
parameter for it (i.e., the class is not required to be a POD type as it did
prior to C++ 11). For example, the program below is well-defined, but Clang
rejects it. GCC 4.8.2 accepts the code as I would expect.

$ cat x.cpp && ./llvm-head-gcc-4.8.2-x86_64-linux/Debug+Asserts/bin/clang -Wall
-std=c++11 -fsyntax-only x.cpp
#include <stdarg.h>

    struct S { S (int); };

    S f (int a, ...) {
        va_list va;
        va_start (va, a);
        S s = va_arg (va, S);
        va_end (va);
        return s;

    S g () {
        return f (0, S (1));
x.cpp:8:27: error: second argument to 'va_arg' is of non-POD type 'S'
        S s = va_arg (va, S);
      expanded from macro 'va_arg'
#define va_arg(ap, type)    __builtin_va_arg(ap, type)
1 error generated.

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