cmd/compile: error when using internal type declarations in generic functions
We hope to support this feature one day, but it doesn't work currently.
Issue a nice error message instead of having the compiler crash.
Update #47631
Change-Id: I0359411410acbaf9a5b9dbb988cd933de1bb8438
Reviewed-on: https://go-review.googlesource.com/c/go/+/364054
Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Dan Scales <danscales@google.com>