What Are the Attributes of Good Diagnostics?

I'm definitely looking for case studies of poor diagnostics. I'd love to see code that was ill-formed or code that was well-formed but just like any compiler or poor diagnostic test cases that you come across, I would love to see those. But I also I'm trying to understand what are the attributes of a good diagnostic. What makes a good diagnostic? There's some way of quantifying what good diagnostics are and like the sort of the attributes, the key properties. And then afterwards, I'll like only print it out of the variant.