That's confusing because "are" ("they're" is a contraction of "they are") is used for the plural. "They is" would be singular, just like "he is" or "she is". It would be ungrammatical for plural "they", but AFIACT is correct for singular "they".

So the unambiguous (but weird sounding to those unused to it) version:

"Is Pat coming to lunch with us?"

"Yes, they is coming in a moment."

> "Yes, they is coming in a moment."

Perhaps one of:

"Yes, they will be here soon / joining us shortly / along later."

would be better.

> "Yes, they will be here soon / joining us shortly / along later."

None of these clarify singular vs plural of 'they.' All of them work whether 'they' refers to one person or multiple.

Good point.

It doesn't solve the pronoun problem but if you haven't heard of them, Grice's Maxims[1] are fun to know about. Basically the idea is that you can generally assume that people you're talking to will follow certain rules, and that allows you to make assumptions. So in your example, you assume that your friend is following the maxim of quantity which allows you to conclude that if there were more people than just Pat, then they would have mentioned the others.

[1] https://www.sas.upenn.edu/~haroldfs/dravling/grice.html

[2] https://en.wikipedia.org/wiki/Cooperative_principle

Yeah, but in practice, "they" is used to describe the plural. Singular they might have been in use, but in the modern context, typically only brings confusion. Not to mention screwing with sentence flow.

Singular they are? they is? Both seem pretty offputting to me.