Structures that use IReference generated in alphabetical order instead of topological order

[cosmin42]

I modified a test to reproduce this issue
test_component.txt - please rename it .txt -> .idl , it is the correspondent to this file https://github.com/microsoft/cppwinrt/blob/master/test/test_component/test_component.idl

The problem:
The below structures cannot be built, the compiler yelds error C2139: 'winrt::test_component::B': an undefined class is not allowed as an argument to compiler intrinsic type trait '__is_trivially_destructible' (compiling source file Simple.cpp)

Swapping the names (A <-> B) solves the problem.

    struct B
    {
        Single a1;
        Windows.Foundation.IReference<Single> a2;
    };

    struct A
    {
        Windows.Foundation.IReference<B> a1;
    };

[kennykerr]

There are all kinds of examples of how structs can cause trouble with C++ due to definition order - unfortunately most of them are not solvable - as a result WinRT APIs just avoid this sort of thing. Here's another example:

namespace Component
{
    namespace A
    {
        struct SimpleA
        {
            Int32 Value;
        };
 
        struct UsesB
        {
            Component.B.SimpleB Value;
        };
    }
 
    namespace B
    {
        struct SimpleB
        {
            Int32 Value;
        };
 
        struct UsesA
        {
            Component.A.SimpleA Value;
        };
    }

There's just no way to practically fix this.

[cosmin42]

What's the recommendation to avoid this? To rename the structures appropriately? To avoid referencing structs or to avoid using structs at all? Is there a known issue section somewhere?

[combinatorial]

@kennykerr isn't a sensible default to use the ordering defined in the IDL? The problem in this case seems to be caused by that ordering being ignored.

[sylveon]

The ordering is lost when the IDL is compiled to a WinMD file (which cppwinrt then uses to generate headers)