That question is about what happens when an illusionist wizard uses Illusory Reality to make a Phantasmal Force image into a real thing. The consensus was that this can't be done, because someone under the effect of Phantasmal Force doesn't know it's an illusion. I'm thinking about a workaround for that.
Phantasmal Force, like many illusion spells, allows a creature who can see the illusion to examine it closely, making an Investigation check.
If the check succeeds, the target realizes that the phantasm is an illusion, and the spell ends.
So if the wizard were to inspect the image and discover that it's an illusion, it goes poof. But consider truesight:
A monster with truesight can, out to a specific range, see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them...
(Now, if you have truesight then the spell can't be cast on you in the first place. So assume that the illusionist casts Phantasmal Force first, and then her friend casts True Seeing on her.)
The question is: Does truesight (1) provide an alternate way to learn that the illusion isn't real, or (2) cause you to successfully inspect the illusion without having to roll (and maybe without having to use an action)?
If (1), then our illusionist can cast Phantasmal Force and believe the illusion is real, then gain truesight and see that it's not real, and then make it real.
If (2), then truesight will simply break the spell.
There's also a possible (3): Even with truesight, the illusionist will be convinced that the illusion is real. Phantasmal Force has the unusual effect of making the target explain away any inconsistencies, so she might, for example, insist that her truesight doesn't work right. I think this is unlikely (since truesight is described in pretty absolute terms) but maybe there's an argument for it.
(Note that my play style doesn't recognize RAW as a source of authority; I'm concerned with applying the rules in a way that tells a coherent story.)