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Better error for data deriving of type synonym/family. Closes #23522

Gregory Gerasev requested to merge uhbif19/ghc:uhbif19/improve-deriving-errors into master

So I was kinda wrong about TcRnIllegalDerivingItem being case of TcRnIllegalClassInst. TcRnIllegalClassInst now only covers TyCons, while TcRnIllegalDerivingItem could be not TyCon at all.

While still this seems strange to me, cuz class instances should be just same in this regard. So this looks like some another code logic duplication somewhere to me, will look on that later. Please take a few moments to address the following points:

CC @RyanGlScott

Closes #23522 (closed)

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