go/types: allow slicing for operands with []byte|string type sets
This is a port of CL 363662 from types2 to go/types. An error message
was adjusted to be on the operand in test data.
Change-Id: I4d2d69976f4f05e0d89ba1c6bf8b3e4cf1a82316
Reviewed-on: https://go-review.googlesource.com/c/go/+/364899
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>