Mark error raising code in PEP 489 init phase as TODO.

Cython/Compiler/ModuleNode.py

@@ -2325,20 +2325,20 @@ class ModuleNode(Nodes.Node, Nodes.BlockNode):
 
         code.put_declare_refcount_context()
         code.putln("#if CYTHON_PEP489_MULTI_PHASE_INIT")
+        # Most extension modules simply can't deal with it, and Cython isn't ready either.
+        # See issues listed here: https://docs.python.org/3/c-api/init.html#sub-interpreter-support
+        code.putln("if (%s) {" % Naming.module_cname)
         # Hack: enforce single initialisation.
-        code.putln("if (%s && %s == %s) return 0;" % (
-            Naming.module_cname,
+        code.putln("if (%s == %s) return 0;" % (
             Naming.module_cname,
             Naming.pymodinit_module_arg,
         ))
-        # # Most extension modules simply can't deal with it, and Cython isn't ready either.
-        # # See issues listed here: https://docs.python.org/3/c-api/init.html#sub-interpreter-support
-        # code.putln("if (%s) {" % Naming.module_cname)
+        # TODO: We should raise an exception here, as long as Cython cannot actually support reinitialisation.
         # code.putln('PyErr_SetString(PyExc_RuntimeError,'
         #            ' "Module \'%s\' has already been imported. Re-initialisation is not supported");' %
         #            env.module_name)
         # code.putln("return -1;")
-        # code.putln("}")
+        code.putln("}")
         code.putln("#elif PY_MAJOR_VERSION >= 3")
         # Hack: enforce single initialisation also on reimports under different names on Python 3 (with PEP 3121/489).
         code.putln("if (%s) return __Pyx_NewRef(%s);" % (