xslt.pxi: calls to _copyXSLT() cast to the wrong type - lxml fails to build with GCC 14

[Victor Stinner]

lxml fails to build with GCC 14. Apparently, GCC 14 switched -Wincompatible-pointer-types warning into an error by default.

xslt.pki:

    new_xslt._context = stylesheet._context._copy()

this code generates:

  __pyx_t_2 = ((PyObject *)__pyx_f_4lxml_5etree_12_XSLTContext__copy(((struct __pyx_obj_4lxml_5etree__BaseContext *)__pyx_v_stylesheet->_context))); if (unlikely(!__pyx_t_2)) __PYX_ERR(5, 684, __pyx_L1_error)

It casts the first argument to the type: "struct __pyx_obj_4lxml_5etree__BaseContext*".

The _copy() function is defined xslt.pki by:

    cdef _BaseContext _copy(self):
        cdef _XSLTContext context
        context = <_XSLTContext>_BaseContext._copy(self)
        context._extension_elements = self._extension_elements
        return context

Cython generates the prototype:

static struct __pyx_obj_4lxml_5etree__BaseContext *__pyx_f_4lxml_5etree_12_XSLTContext__copy(struct __pyx_obj_4lxml_5etree__XSLTContext *__pyx_v_self) { ... }

The first parameter type (self) is: "struct __pyx_obj_4lxml_5etree__XSLTContext*".

XSLT._context is defined in xslt.pki with:

    @cython.no_gc_clear
    cdef class XSLT:
        cdef _XSLTContext _context

Well, I see different options:

* Change XSLT._context type to type expected by _copy(): _BaseContext
* Cast XSLT._context to _BaseContext somehow
* Change _copy() parameter type to _XSLTContext?

Extract of GCC 13 compiler warnings if I pass manually -Wincompatible-pointer-types flag:
---
$ gcc -fno-strict-overflow -Wsign-compare -DDYNAMIC_ANNOTATIONS_ENABLED=1 -DNDEBUG -fexceptions -fcf-protection -fexceptions -fcf-protection -fexceptions -fcf-protection -fPIC -DCYTHON_CLINE_IN_TRACEBACK=0 -I/usr/include/libxml2 -Isrc -Isrc/lxml/includes -I/tmp/build-env-9r2e6zhz/include -I/usr/include/python3.12 -c src/lxml/etree.c -o etree.o -Wincompatible-pointer-types

(...)

src/lxml/etree.c: In function '__pyx_pf_4lxml_5etree_4XSLT_16__call__':
src/lxml/etree.c:225042:73: warning: passing argument 1 of '__pyx_f_4lxml_5etree_12_XSLTContext__copy' from incompatible pointer type [-Wincompatible-pointer-types]
225042 | __pyx_t_2 = ((PyObject *)__pyx_f_4lxml_5etree_12_XSLTContext__copy(((struct __pyx_obj_4lxml_5etree__BaseContext *)__pyx_v_self->_context))); if (unlikely(!__pyx_t_2)) __PYX_ERR(5, 543, __pyx_L9_error)
       | ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       | |
       | struct __pyx_obj_4lxml_5etree__BaseContext *
src/lxml/etree.c:222496:138: note: expected 'struct __pyx_obj_4lxml_5etree__XSLTContext *' but argument is of type 'struct __pyx_obj_4lxml_5etree__BaseContext *'
222496 | static struct __pyx_obj_4lxml_5etree__BaseContext *__pyx_f_4lxml_5etree_12_XSLTContext__copy(struct __pyx_obj_4lxml_5etree__XSLTContext *__pyx_v_self) {
       | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~

src/lxml/etree.c: In function '__pyx_f_4lxml_5etree__copyXSLT':
src/lxml/etree.c:226859:71: warning: passing argument 1 of '__pyx_f_4lxml_5etree_12_XSLTContext__copy' from incompatible pointer type [-Wincompatible-pointer-types]
226859 | __pyx_t_2 = ((PyObject *)__pyx_f_4lxml_5etree_12_XSLTContext__copy(((struct __pyx_obj_4lxml_5etree__BaseContext *)__pyx_v_stylesheet->_context))); if (unlikely(!__pyx_t_2)) __PYX_ERR(5, 684, __pyx_L1_error)
       | ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       | |
       | struct __pyx_obj_4lxml_5etree__BaseContext *
src/lxml/etree.c:222496:138: note: expected 'struct __pyx_obj_4lxml_5etree__XSLTContext *' but argument is of type 'struct __pyx_obj_4lxml_5etree__BaseContext *'
222496 | static struct __pyx_obj_4lxml_5etree__BaseContext *__pyx_f_4lxml_5etree_12_XSLTContext__copy(struct __pyx_obj_4lxml_5etree__XSLTContext *__pyx_v_self) {
       | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~
---

I attach the full output as attached warnings.log.

[Victor Stinner (vstinner)]

Example of commands to get the warnings (tested on Fedora with GCC 13):

$ git clean -fdx

$ CFLAGS="-Wincompatible-pointer-types" python3 setup.py build_ext --warnings 2>&1|tee build.log

$ grep copy build.log
(...)
src/lxml/etree.c:225039:73: warning: passing argument 1 of ‘__pyx_f_4lxml_5etree_12_XSLTContext__copy’ from incompatible pointer type [-Wincompatible-pointer-types]
(...)

[Miro Hrončok (churchyard)]

This is possibly caused by https://github.com/cython/cython/issues/2747