Two interface types that are assignable don't have to be identical;
specifically, if they are defined types, they can be different
defined types. If those defined types specify type parameters which
are never used, do not infer a type argument based on the instantiation
of a matching defined type.
Adjusted three existing tests where we inferred type arguments incorrectly.
Fixes#60377.
Change-Id: I91fb207235424b3cbc42b5fd93eee619e7541cb7
Reviewed-on: https://go-review.googlesource.com/c/go/+/498315
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
