go/types, types2: typeparams.IndexExpr must not be an ast.Expr
The typeparams.IndexExpr wrapper type was added as a compatibility layer
to make the go/types code symmetric with types2. However, this type
incidentally implemented the ast.Expr interface, leading to the
accidental misuse that led to golang/go#63933.
Fix this minimally for now, though leave a TODO that this old
compatibility shim really needs to be eliminated.
Also fix a case in types2 where operand.expr was set to a typed nil.
Fixes golang/go#63933
Change-Id: I180d411e52f795a8322ecce6ed8649e88af1c63b
Reviewed-on: https://go-review.googlesource.com/c/go/+/554395 Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>