"Errors" may be emitted, as a side effect, but correspond to a tree that isErroneous.

My code editor correlates the diagnostic with a source position, and that is where the red squiggle appears.

But warnings do not have a corresponding tree state. They are only positioned diagnostics (where the yellow squiggle appears).

I can test if tree.isErroneous but not tree.isDubious. Why is that? Are warnings fundamentally second-class annotations?

I just had a minor case where I might have tested isDubious to avoid warning twice.

Trees may be deemed in error internally, but not reported because other strategies are tried (other implicits, etc). There is no such mechanism for warnings.