Behavior of HasType and HasInterface is counter-intuitive #145

Both of these functions call If which does a traversal of the causal chain, it does not traverse the full tree. Specifically, If internally calls UnwrapOnce and doesn't handle the multi-error case, whereas functions like Is and As separately handle the multi-error case.

This leads to counter-intuitive behavior; you can have a value x of type T, and errors.Is(err, x) may be true, but errors.HasType(err, T{}) may fail.

I noticed this behavior while trying to add property-based tests to better understand the behavior of HasType here. https://github.com/sourcegraph/sourcegraph/pull/62992

It would be valuable to either:

• Change the implementation of If to traverse the full tree
• OR Add a separate function which does a tree traversal (not just the "causal chain") and use that from HasType and HasInterface
• OR Add a cautionary warning to HasType and HasInterface's docs which describe the behavior in the presence of multi-errors.