go/types, types2: always try inference over methods when possible
During type inference, when comparing type parameters against their
constraints, if a type argument is completely known it must implement
its constraint. In this case, always unify the type argument's methods
against the constraint methods, if any.
Before this CL, this step was only attempted if the constraint had no
core type. That left information unused which led to type inference
failures where it should have succeeded.
Fixes #66751.
Change-Id: I71e96b71258624212186cf17ec47e67a589817b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/617896 Reviewed-by: Tim King <taking@google.com> Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
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