cmd/compile/internal/types2: refactor untyped conversions

Based on https://golang.org/cl/284256 for go/types.
Brings this code more in line with go/types.

Adjusted various tests to match new error messages which
generally are now better: for assignment errors, instead
of a generic "cannot convert" we now say "cannot use"
followed by a clearer reason as to why not.

Major differences to go/types with respect to the changed
files:

- Some of the new code now returns error codes, but they
  are only used internally for now, and not reported with
  errors.

- go/types does not "convert" untyped nil values to target
  types, but here we do. This is unchanged from how types2
  handled this before this CL.

Change-Id: If45336d7ee679ece100f6d9d9f291a6ea55004d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/302757
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/src/cmd/compile/internal/types2/api.go b/src/cmd/compile/internal/types2/api.go
index 30f0430ff1..acb196a336 100644
--- a/src/cmd/compile/internal/types2/api.go
+++ b/src/cmd/compile/internal/types2/api.go
@@ -415,7 +415,8 @@ func AssertableTo(V *Interface, T Type) bool {
 // AssignableTo reports whether a value of type V is assignable to a variable of type T.
 func AssignableTo(V, T Type) bool {
 	x := operand{mode: value, typ: V}
-	return x.assignableTo(nil, T, nil) // check not needed for non-constant x
+	ok, _ := x.assignableTo(nil, T, nil) // check not needed for non-constant x
+	return ok
 }
 
 // ConvertibleTo reports whether a value of type V is convertible to a value of type T.