Eugene, I'd like to offer a long +1 to (carefully reviewed) breaking changes here. I know you weren't asking me, but I think I can contribute my 2 cents here.
The following has some assumption on the change: it should make the actual Scalac behavior simpler to describe. Try documenting the current behavior (bugs included), try documenting the behavior with the patch, verify the latter is much simpler and actually understandable; in particular, the latter should be more declarative.
Because of such bugs, Scalac typechecker's behavior feels random. To see a discussion of them, look no further than this blog post:
(I also wonder whether something in there has to do with this bug, maybe seq2richseq? There's a comment from Adriaan Moors about another relation. But don't take me too seriously here, the details are a vague recollection from ages ago; my point is about why such bugs are bad).
Moreover, it seems that the code which will break is (arguably) buggy anyway, even though without a fixed Scalac this is probably hard to see.
And changes which fix implicit resolution and break code have gone in as late as 2.10 (IIRC). If wanted, one could support the transition somehow: make the new behavior optional and controlled by an option, or add a warning in 2.11 for code which will break (hard) and make the change for 2.12 (though I'd like things to go faster).
Overall, I think we need a way to fix bugs which make (buggy) code compile. C++ compilers often broke buggy code, I don't think Scala needs better bug-to-bug compatibility.
(Sorry for the length).