[Bug c++/82893] New: Bad diagnostic on negative sized array

[barry.revzin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82893

            Bug ID: 82893
           Summary: Bad diagnostic on negative sized array
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

Consider this example:

struct Foo {
    int n;
};
struct Bar {
    double n;
    bool b;
};
template <typename T>
struct Baz : public T {
    char pad[8 - sizeof(T)];
};

int main() {
    static_assert(sizeof(Baz<Foo>) == 8, "");
    static_assert(sizeof(Baz<Bar>) == 8, "");
}

The diagnostic that every gcc yields (on basically every version) is:

prog.cc:15:5: error: non-constant condition for static assertion
     static_assert(sizeof(Baz<Bar>) == 8, "");
     ^~~~~~~~~~~~~

This is very misleading and unhelpful. It'd be nice if it were possible to get
a better diagnostic that points to the cause of the problem. clang, for
instance, yields:

prog.cc:10:14: error: array is too large (18446744073709551608 elements)
    char pad[8 - sizeof(T)];
             ^~~~~~~~~~~~~
prog.cc:15:19: note: in instantiation of template class 'Baz<Bar>' requested
here
    static_assert(sizeof(Baz<Bar>) == 8, "");
                  ^