Mangle cname for closures in derived c class (GH-2990)

Cython/Compiler/ParseTreeTransforms.py

@@ -2715,9 +2715,12 @@ class CreateClosureClasses(CythonTransform):
             node.needs_outer_scope = True
             return
 
+        # entry.cname can contain periods (eg. a derived C method of a class).
+        # We want to use the cname as part of a C struct name, so we replace
+        # periods with double underscores.
         as_name = '%s_%s' % (
             target_module_scope.next_id(Naming.closure_class_prefix),
-            node.entry.cname)
+            node.entry.cname.replace('.','__'))
 
         entry = target_module_scope.declare_c_class(
             name=as_name, pos=node.pos, defining=True,

tests/run/closure_in_derived_class_T2967.pyx

+# mode: run
+# tag: closures
+# ticket: 2967
+
+cdef class BaseClass:
+    cdef func(self):
+        pass
+cdef class ClosureInsideExtensionClass(BaseClass):
+    """
+    >>> y = ClosureInsideExtensionClass(42)
+    >>> y.test(42)
+    43
+    """
+    cdef func(self):
+        a = 1
+        return (lambda x : x+a)
+    def test(self, b):
+        return self.func()(b)