Calling a const function rather than its non-const version

It doesn't matter whether Data::x is a constant function or not. The operator being called belongs to container<Data> class and not Data class, and its instance is not constant, so non-constant operator is called. If there was only constant operator available or the instance of the class was constant itself, then constant operator would have been called.

test is a non-const object, so the compiler finds the best match: The non-const version. You can apply constness with static_cast though: static_cast<const testType&>(test)->x();

EDIT: As an aside, as you suspected 99.9% of the time you think you've found a compiler bug you should revisit your code as there's probably some weird quirk and the compiler is in fact following the standard.

If you have two overloads that differ only in their const-ness, then the compiler resolves the call based on whether *this is const or not. In your example code, test is not const, so the non-const overload is called.

If you did this:

testType test;
const testType &test2 = test;
test2->x();

you should see that the other overload gets called, because test2 is const.