go/types: Identical: document the need for consistent symbols

Fixes #66690
Updates #57497

Change-Id: I3d8f48d6b9baae8d5518eefeff59c83b12728cf5
Reviewed-on: https://go-review.googlesource.com/c/go/+/577015
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>

Author: adonovan

src/cmd/compile/internal/types2/api_predicates.go

@@ -70,6 +70,18 @@ func Satisfies(V Type, T *Interface) bool {
 
 // Identical reports whether x and y are identical types.
 // Receivers of [Signature] types are ignored.
+//
+// Predicates such as [Identical], [Implements], and
+// [Satisfies] assume that both operands belong to a
+// consistent collection of symbols ([Object] values).
+// For example, two [Named] types can be identical only if their
+// [Named.Obj] methods return the same [TypeName] symbol.
+// A collection of symbols is consistent if, for each logical
+// package whose path is P, the creation of those symbols
+// involved at most one call to [NewPackage](P, ...).
+// To ensure consistency, use a single [Importer] for
+// all loaded packages and their dependencies.
+// For more information, see https://github.com/golang/go/issues/57497.
 func Identical(x, y Type) bool {
 	var c comparer
 	return c.identical(x, y, nil)