go/types, types2: AssertableTo is undefined for generalized interfaces

Document that AssertableTo is undefined (at least for 1.18) if
the first argument is a generalized interface; i.e., an interface
that may only be used as a constraint in Go code.

Still, implement it as we might expect it to be defined in the
future, to prevent problems down the road due to Hyrum's Law.

While at it, also removed the internal flag forceStrict and its
one use in Checker.assertableTo; forceStrict was never enabled
and if it would have been enabled, the behavior would not have
been correct.

Change-Id: Ie4dc9345c88d04c9640f881132154a002db22643
Reviewed-on: https://go-review.googlesource.com/c/go/+/383917
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

0 files changed, 0 insertions, 0 deletions