go/types: return an error from Instantiate on incorrect len(targs)
Instantiate already returns an error when validation fails. Panicking on
an incorrect number of type arguments means that callers must both
pre-validate the number of type arguments and handle resulting errors.
Returning an error rather than panicking allows eliminating
pre-validation at the call-site.
Also update the Instantiate docstring to correct some stale/inaccurate
information, and to clarify its behavior more precisely.
Updates #47916
Change-Id: I997ef30b3486760a90b0db4c3ea7111280d74a81
Reviewed-on: https://go-review.googlesource.com/c/go/+/363635
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>