[Bug c/100124] New: Why is "flexible array member '...' in an otherwise empty '...'" an issue?

["gcc at behdad dot org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100124

            Bug ID: 100124
           Summary: Why is "flexible array member '...' in an otherwise
                    empty '...'" an issue?
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc at behdad dot org
  Target Milestone: ---

Hi,

In HarfBuzz we extensively use the Struct Hack [0]. Up until now we've used
"array[1]" for that. This upsets ubsan [1]. So I'm looking to replace it with
flexible arrays on compilers that support it.

However, trying to do that I get error if the flexible-array is the only
member. Clang has no issues with it, but gcc gives me: "flexible array member
'...' in an otherwise empty '...'".


```
struct UnsignedArray {
  int array[];
};
```

I've seen:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69550
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71742

which suggests that disallowing flexible arrays in structs that have no other
named member is intentional. But I cannot find the reason. Can someone please
help me understand, and suggest a solution here?

Note that if I use `int array[0]` instead, then gcc warns everytime in code we
access that array with a constexpr value, like `array[0]` when we know it's
safe to do. That is, gcc seems to treat `int array[0]` literally, not as a
flexible array.

Thank you.


[0] http://c-faq.com/struct/structhack.html
[1] https://github.com/harfbuzz/harfbuzz/issues/2953