go/types: systematic detection of missing instantiation
This is a port of CL 346471 to go/types. Additionally, CheckExpr was
updated for the new API to explicitly allow generic expressions.
The error messages in issue39634.go2 are different because go/parser
produces an IndexExpr with BadExpr index value, for backward
compatibility.
Change-Id: I725926de183a016381513fe0e750d1280688ce29
Reviewed-on: https://go-review.googlesource.com/c/go/+/347391
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>