Ambiguous tests in test_hpytype_legacy.py

[mattip]

The code in test_htype.py has tests like

    def test_simple_type(self):
        mod = self.make_module("""
            static HPyType_Spec Dummy_spec = {
                .name = "mytest.Dummy",
                .itemsize = 0,
                .flags = HPy_TPFLAGS_DEFAULT | HPy_TPFLAGS_BASETYPE,
                @DEFAULT_SHAPE
            };

            @EXPORT_TYPE("Dummy", Dummy_spec)
            @INIT
        """)

When run in test_hpytype.py then @DEFAULT_SHAPE is an empty comment "/* default object shape */" and everything is fine. But when these tests are rerun in test_hpytype_legacy.py, then @DEFAULT_SHAPE becomes .builtin_shape = HPyType_BuiltinShape_Legacy.

As I understand the intention of using the legacy shape is that the type can be used via the C-API, so it needs to become a heap type object. But the preamble to add #include <Python.h> is missing, and the basicsize is 0 (not declared). I guess I can work around the missing include file somehow, but it is not clear to me if a type spec can declare legacy shape without declaring a basicsize. Thoughts? Should this Just Work (i.e. when creating a type from the spec, note the legacy flag and allocate a heap type object as the storage)?