The required changes were surprisingly minimal (I was hoping for a net
code deletion) because ssa is essentially doing its own type inference
for the three value,ok operators---and it continues to have to do so.
To see why, consider:
var i interface{}; var ok bool
i, ok := (map[string]string)(nil)[""]
Before, go/types inferred interface{} for the RHS of the assignment,
and now it infers (interface{}, bool), yet neither of these is what
ssa needs: it needs to know that the map values were strings so
that it can emit a lookup of the right type followed by a conversion
from string to interface{}.
TBR=gri
CC=golang-dev
https://golang.org/cl/11988044
e7016436a6