Don't arbitrarily choose one upper bound for hidden captured region error message
You could argue that the error message is objectively worse, even though it's more accurate. I guess we could also add a note explaining like "cannot capture the intersection of two regions" or something, though I'm not sure if that is confusing due to being totally technical jargon.
This addresses the fact that #128752 says "add `+ 'b`" even though it does nothing to fix the issue. It doesn't fix the issue's root cause, though.
r? `@spastorino`
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