Fix typo that resulted in comparison-to-self

[isabelmu]

This line was introduced in commit 843db01, rebased from eddyb's c5f6f84.

I don't know whether this fixes anything in practice, but it seems like the changed version was what was intended originally.

r? @eddyb

[eddyb]

The bug would allow Box<Trait> to be cast to Box<DifferentTrait>, if not for the subtyping check later.
That is, Trait<A> vs Trait<B> is checked later anyway, all this does is have a superfluous candidate.
What it could affect is ambiguity, but Unsize is built-in anyway, so I can't even think of a testcase.

Thanks for finding this! Even if the check is ultimately pointless, still better to keep it correct 😆.

@bors r+ rollup

[bors]

📌 Commit f9cb7df has been approved by eddyb

[isabelmu]

Thanks for explaining :-)

[bors]

⌛ Testing commit f9cb7df with merge 1e5dab1...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-rustbuild
Approved by: eddyb
Pushing 1e5dab1 to master...