[llvm-bugs] [Bug 24711] New: Struct size is 0 even if the compiler emites "warning: struct has size 0 in C, size 1 in C++ [-Wc++-compat]"

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=24711

            Bug ID: 24711
           Summary: Struct size is 0 even if the compiler emites "warning:
                    struct has size 0 in C, size 1 in C++ [-Wc++-compat]"
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: douglas_yung at playstation.sony.com
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org
    Classification: Unclassified

Consider the following code:

/*** test.c ***/
#include <stdio.h>

struct S {
  char tmp[0];
};

struct T {
  struct S v01;
  struct S v02;
};

int main(void) {
  printf("%lu\n", sizeof(struct S));
  printf("%lu\n", sizeof(struct T));
  return 0;
}
/*****/

If you compile this as a .c file on x64 linux targeting x64 linux, using
-Wc++-compat, you get the following warnings:

test.c:3:1: warning: struct has size 0 in C, size 1 in C++ [-Wc++-compat]
struct S {
^
test.c:7:1: warning: struct has size 0 in C, non-zero size in C++
[-Wc++-compat]
struct T {
^

But if rename the file to be .cpp and recompile, you find that both of those
warnings are not true. The size of both S and T are reported to be 0, which
does at least seem to be consistent with what gcc produces.

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