go/types: missing "unused var" error due to incorrect scope computation

[alandonovan]

% cat a.go
package main

type I interface {
f() chan int
}

func f(i I) {
select {
case <-chan int(nil):
var i I // unused, but no error
case <-i.f(): // delete this case, and you'll see the error
}
}
% gotype a.go && echo PASS
PASS

The real problem here is not the failure to report an error, but a mistake in the scope calculation.
I think go/types is treating the reference to i in the second case as if it is within the scope of the 'var i I' declaration, in other words, it's resolving to the wrong i.

I think this explains SSA bug #9569 too.

[alandonovan]

Fixed by https://go-review.googlesource.com/#/c/2655/