Turn the redeclaration of attributes as C methods into an error since it...

Turn the redeclaration of attributes as C methods into an error since it currently leads to difficult to predict behaviour. (#4661)

Cython/Compiler/Symtab.py

@@ -2439,7 +2439,8 @@ class CClassScope(ClassScope):
             cname = punycodify_name(c_safe_identifier(name), Naming.unicode_vtabentry_prefix)
         if entry:
             if not entry.is_cfunction:
-                warning(pos, "'%s' redeclared  " % name, 0)
+                error(pos, "'%s' redeclared " % name)
+                entry.already_declared_here()
             else:
                 if defining and entry.func_cname:
                     error(pos, "'%s' already defined" % name)

tests/errors/redeclaration_of_var_by_cfunc_T600.pyx

+# ticket: 600
+# mode: error
+
+cdef class Bar:
+    cdef list _operands
+
+    cdef int _operands(self):
+        return -1
+
+
+_ERRORS = """
+7:9: '_operands' redeclared
+5:14: Previous declaration is here
+"""