…must terminate (bug fix)

When types2.Instantiate is called externally, no *Checker is provided and
substitution doesn't have access to Checker.typMap; and instantiation of
recursive generic types leads to an infinite recursion in subst.

There was a local subster.cache but it was only set and never used.
Replaced subster.cache with subster.typMap, which is set to the global
Checker.typMap if available, and set to a local map otherwise. This
prevents such infinite recursions. Added a simple test.

More generally, because we don't have a global type map for external
instantiations, instantiating the same type twice, independently but
with the same type arguments, will result in two different types. This
is not correct. We need to provide some form of context for external
instantiations (which means the importers). This is a separate but
related issue which is not yet addressed (filed #47103).

Change-Id: I541556c677db54f7396fd0c88c7467894dfcf2e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/333383
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

…ate with Checker.Instantiate

Allow Checker.Instantiate to work with a nil *Checker receiver
(for now). This opens the door to passing in a *Checker at all
times.

Also, added a verify flag to Instantiate, InstantiateLazy, and
instance, to be able to control if constraint satisfaction should
be checked or not.

Removed types2.Instantiate.

For #47103.

Change-Id: Ie00ce41b3e50a0fc4341e013922e5f874276d282
Reviewed-on: https://go-review.googlesource.com/c/go/+/333569
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

Extend the type checking context to allow de-duplicating *Signature
instances, in addition to *Named instances.

Naively we would deduplicate instances of different-but-identical origin
*Signature types. That may be OK, but it seems a bit strange to get the
same signature when instantiating two different functions. For now,
differentiate *Signature types by prepending a unique identifier for the
origin pointer, thus guaranteeing that instances de-duplicated if they
come from the exact same (pointer identical) origin type.

Updates #47103

Change-Id: I93cc3cacad195267fe0a5801f9c5a3b1e61eb907
Reviewed-on: https://go-review.googlesource.com/c/go/+/362801
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>