Make vtable order of extension types with fused methods only dependant on the original declaration order (e.g. in the .pxd file).

Previously, fused methods were specialised and expanded on first use, which lead to an arbitrary order in the vtable.
Also fixes compile failures when inheriting from base types with fused cdef methods.
Fixes #1873.

1 files changed, 7 insertions, 0 deletions

diff --git a/Cython/Compiler/ParseTreeTransforms.py b/Cython/Compiler/ParseTreeTransforms.py
index 61625e842..7ee9861a8 100644
--- a/Cython/Compiler/ParseTreeTransforms.py
+++ b/Cython/Compiler/ParseTreeTransforms.py
@@ -1535,6 +1535,13 @@ class ForwardDeclareTypes(CythonTransform):
     def visit_CClassDefNode(self, node):
         if node.class_name not in self.module_scope.entries:
             node.declare(self.module_scope)
+        # Expand fused methods of .pxd declared types to construct the final vtable order.
+        type = self.module_scope.entries[node.class_name].type
+        if type is not None and type.is_extension_type and not type.is_builtin_type and type.scope:
+            scope = type.scope
+            for entry in scope.cfunc_entries:
+                if entry.type and entry.type.is_fused:
+                    entry.type.get_all_specialized_function_types()
         return node