The loading of the base type in typ0() may cause s.Def to be defined for
the instantiated type, so load the base type before checking s.Def.
Fixes#50486
Change-Id: Ic039bc8f774dda534f4ccd1f920220b7a10dede6
Reviewed-on: https://go-review.googlesource.com/c/go/+/377094
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>