Maybe it's just me, but the errors that needed updating in the prior examples seems worse / harder to interpret now? I'd like to be convinced that I'm wrong though, that's just my initial reaction from looking at the diffs. (I realise this in in the "hint" part of the error too, so the main part is unaffected at least).

No, I agree that they aren't great hints. But the hints being replaced were literally copies of the main error message, so this has to be some sort of improvement over that at least.

I was separately thinking that these hints might be improved if we used the post-substitution types instead of pre-substitution, but I thought I'd see how #4163 shakes out before changing anything in that part of the code.

No, I agree that they aren't great hints. But the hints being replaced were literally copies of the main error message, so this has to be some sort of improvement over that at least.

I was separately thinking that these hints might be improved if we used the post-substitution types instead of pre-substitution, but I thought I'd see how #4163 shakes out before changing anything in that part of the code.

It looks like there was a minor readability improvement (only two .out files changed). I'm not sure if they'll have any impact here.

Are there any other objections to this? Or can we get another approval and get this merged?