test: add test case for type parameter method indexing
When types2 type checks a method expression or method value that
selects a type parameter method, the Selection.Index is indexed based
on the method's index within the type parameter's constraint
interface.
However, with a fully-stenciled implementation, naively using the
index would result in picking a method from the corresponding type
argument's full method set, which could select a different method.
Unified IR currently avoids this because it selects methods based on
name, not index; but experimenting with index-based selection revealed
that there are no test cases that would have caught this failure case.
Change-Id: Idbc39e1ee741714203d4749e47f5bc015af25020
Reviewed-on: https://go-review.googlesource.com/c/go/+/421815
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: David Chase <drchase@google.com>