cmd/compile: fix handling of partially inferred type arguments
In the case of partially inferred type arguments, we need to use the
IndexExpr as the key in g.info.Inferred[] rather than the CallExpr.
Added an extra fromStrings1 call in the settable.go test that tests
partially inferred type arguments. This new call uses a new concrete
type SettableString as well.
I also added another implementation fromStrings3 (derived from a go2go
tests) that typechecks but intentionally causes a panic.
Change-Id: I74d35c5a741f72f37160a96fbec939451157f392
Reviewed-on: https://go-review.googlesource.com/c/go/+/300309
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Dan Scales <danscales@google.com>
Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>