go/types, types2: use inexact unification when unifying against core types
Follow-up on CL 498955 which introduced a unification mode, to be used
to control the precision of unification of element types (CL 498895):
When unifying against core types of unbound type parameters, we must
use inexact unification at the top (irrespective of the unification mode),
otherwise it may fail when unifying against a defined type (core types
are always underlying types).
No specific test case (I have not been able to create one yet).
Change-Id: Ie15e98f4b9e9fb60d6857d34b03d350ebbf0375e
Reviewed-on: https://go-review.googlesource.com/c/go/+/501302
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Robert Findley <rfindley@google.com> Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
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