Question

why should a circular saw that does not have an upper blade guard never be used?
the trigger lock will not work properly.
the safety features will be disabled.
the exposed blade poses a danger.

Explanation:

A circular saw without an upper blade guard exposes the blade. An exposed blade can cause cuts or injuries to the user. Safety features like blade guards are crucial in power tools to prevent accidents. The trigger lock not working properly is not related to the absence of an upper blade guard. Saying safety features will be disabled is too broad; the main specific danger is the exposed blade.

Answer:

The exposed blade poses a danger.