Jon Cryer In The Dark About Demi's Addiction ... Even While Dating!!!
Jon Cryer's opening up about his relationship with Demi Moore back in the day ... revealing he had no idea about her drug addiction while they were together.
The actor -- who starred alongside Moore in the hit 1984 flick "No Small Affair" -- tells People -- briefly dated his costar while making the classic movie ... but, JC didn't realize she was using until about two weeks before wrap.
The way Jon explains it ... their tryst ended fairly quickly, and he remembers her as having a charismatic spark to her -- all of which completely fooled him.