[REG2.065a][enh] Issue 11946 - "need 'this' to access member" when passing field to template parameter

https://d.puremagic.com/issues/show_bug.cgi?id=11946

If alias parameter takes nested argument but its context pointer is not really accessible, instantiated template should not have implicit enclosing context.

This is also a small language enhancement, and some ever unnecessarily rejected code will be accepted.