Struct size is 0 even if the compiler emits "warning: struct has size 0 in C, size 1 in C++ [-Wc++-compat]"

[dyung]

Bugzilla Link	24711
Version	trunk
OS	Linux
CC	@DougGregor

Extended Description

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.

[dyung]

This was done using an x64 linux compiler built from r246826.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (dyung)

| | | | --- | --- | | Bugzilla Link | [24711](https://llvm.org/bz24711) | | Version | trunk | | OS | Linux | | CC | @DougGregor |

Extended Description

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.