[Cython] Big backwards compatibility problem with fused types

[Stefan Behnel]

Hi devs,

this is really bad:

https://github.com/cython/cython/pull/1873

The problem is that cdef functions with fused types are expanded
arbitrarily on use, and their Entries removed and re-appended to the list
of cdef function entries. But that list also defines the order of the
entries in the vtable!

Currently, when a method with fused types is defined in a .pxd file, it is
the implementation order in the .pyx (!) file that determines the order in
the vtable. Even worse, for modules that cimport from that .pxd file, it is
the order in which these methods are *called* that determines the
corresponding vtable on the other side, as the entries are expanded at
first use, i.e. at their first occurrence in the code, and appended to the
current list of cdef functions at this (arbitrary) point.

What this means is that there is no way to safely infer the vtable order of
an extension type with fused methods from a .pxd file.

Now, the correct way to handle this would have been to *replace* the
original fused entry with the expanded list of specialised entries. But
it's too late for that now. All existing modules out there would need to
get recompiled if we changed their vtable order, at least if they use fused
any types methods anywhere in their hierarchy.

However, I think we can assume code that uses a different method order in
the .pxd and .pyx files to be broken already, so a way out of this misery
would be to make the .pxd order the one and only source, with the twist
that we must keep the "delete and re-append" algorithm to keep the order
that existing translated modules are using.

Thus the proposal:

- we switch to a scheme now that is only defined by the first declaration
order, i.e. the order in the .pxd file if there is one.

- we make sure all fused methods are expanded in that same order, but
continue to follow all non-fused methods in the vtable.

- we break all code now that uses a different order of fused methods in
their pyx and pxd files

Thoughts?

Stefan