Based on https://golang.org/cl/284256 for go/types.
Brings this code more in line with go/types.
Adjusted various tests to match new error messages which
generally are now better: for assignment errors, instead
of a generic "cannot convert" we now say "cannot use"
followed by a clearer reason as to why not.
Major differences to go/types with respect to the changed
files:
- Some of the new code now returns error codes, but they
are only used internally for now, and not reported with
errors.
- go/types does not "convert" untyped nil values to target
types, but here we do. This is unchanged from how types2
handled this before this CL.
Change-Id: If45336d7ee679ece100f6d9d9f291a6ea55004d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/302757
Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>