Declaring is_message in namespace rosidl_generator_traits.

[shoeffner]

Resolves issues with older GCC versions, as described in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56480.

Resolves #479.

#479 seems to be a bug in gcc. The original bug report states that:

namespace Foo 
{
    template <typename T>
    struct Meow
    {
    };
}

template <>
struct Foo::Meow<int>
{
};

failed, while

The code is accepted if a declaration (without definition) of the specialization is placed in the namespace.

So changing

rosidl/rosidl_generator_cpp/test/test_traits.cpp

Lines 31 to 32 in 76c536e

	template<>
	struct rosidl_generator_traits::is_message<Message2>: std::true_type {};

to

namespace rosidl_generator_traits
{

template<>
struct is_message<Message2>: std::true_type {};

}

resolves #479.

[dirk-thomas]

What platform are you targeting?

Even Ubuntu Bionic (which is not even officially supported by Rolling / Foxy) provides gcc 7.x which seems to have this issue already address.

[shoeffner]

I had the issue appearing on Debian 9 (stretch), where gcc is still version 6.4 (https://packages.debian.org/stretch/gcc). @gramss mentioned in #479 Debian 4.9, which I assume is also stretch because its kernel version is 4.9. However, I am aware Debian is not supported and submitted the PR as suggested in #479 (in fact, for stretch I didn't get the Qt dependencies for other packages of foxy to compile and thus am using melodic until there is a chance to change the PC to a supported Ubuntu - hopefully soon).

…

On Mon, Aug 10, 2020, 19:13 Dirk Thomas ***@***.***> wrote: What platform are you targeting? Even Ubuntu Bionic (which is not even officially supported by Rolling / Foxy) provides gcc 7.x which seems to have this issue already address. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#512 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAOAOD4IHQPBYGL65M7YPDDSAATCDANCNFSM4PYSU3TA> .

[dirk-thomas]

I had the issue appearing on Debian 9 (stretch), where gcc is still version
6.4

Thanks for the context. Since the patch is trivial I think it makes sense to make it work for that platform (even if not officially targeted).

[nuclearsandwich]

The affected test does not exist in Dashing so there is nothing to backport there.