The type checker does not currently report the set of pairs of
types that are tested for assignability (though gri and I
agree that it should). This information is useful for many
applications. For example, refactoring tool needs to know the
minimal set of interface satisfaction constraints that must be
preserved during a refactoring.
This package is a stopgap measure to deduce the same
information using another pass. Unlike go/types, it is not
robust against inputs containing type errors.
LGTM=sameer
R=gri, sameer
CC=golang-codereviews
https://golang.org/cl/136470043
