[cfe-dev] Handling of flexible arrays vs. old-style zero-length arrays.

[Andrea Di Biagio]

This is the correct reproducible for the assertion failure.

// ---
struct A {
  int a0[0];
};

struct B : private A {  // assertion failure!
  int b;
};

B bb;
// ---


On Mon, Sep 22, 2014 at 5:02 PM, Andrea Di Biagio
<andrea.dibiagio at gmail.com> wrote:
> Hi all,
>
> If I pass the following code to clang:
>
> // ---
> struct A {
>   int a0[];
> };
>
> struct B : public A {
>   int b;
> };
> // ---
>
> the compiler emits the following error:
> "error: base class 'A' has a flexible array member".
> I guess that is expected because (quoting the comment in
> Sema/SemaDeclCXX) a class which contains a flexible array member is
> not suitable for use as a base class.
>
> However, if I replace the flexible array declaration in 'struct A'
> with another flexible array declaration where the array length is
> explicitly set to zero, then the compiler no longer emits an error
> (see below).
>
> // ---
> struct A {
>   int a0[0];
> };
>
> struct B : public A {  // no error reported.
>   int b;
> };
> // ---
>
> Shouldn't the compiler error in this case as well?
>
> Also, if B is a non-POD type, then clang asserts in
> CGRecordLayoutBuilder.cpp within method 'clipTailPadding'.
> In the attempt to lowering B, it triggers the assertion failure
> (around line 525):
> 'Prior->Kind == MemberInfo::Field && !Prior->FD && "Only storage
> fields have tail padding!'.
>
> {reproducible}
> // ---
> struct A {
>   int a0[0];
> };
>
> struct B : private A {  // assertion failure!
>   int b;
> };
> // ---
> {end-of-reproducible}
>
> (I plan to raise a bug for this one).
>
> In Sema/SemaDeclCXX.cpp we check if
> 'CXXBaseDecl>hasFlexibleArrayMember()' and in case, we emit a
> diagnostic error.
> Should clang also check if the code is using a zero-length array
> declaration and in case emit an error?
>
> Thanks,
> -Andrea