[dev.go2go] go/types: don't crash in assignment due to prior errors

Addresses crash #12 of 39634.

Updates #39634.

Change-Id: Ie67545d21b6a1e80fcdec1b2eb101b69c25e2952
Reviewed-on: https://go-review.googlesource.com/c/go/+/245738
Reviewed-by: Robert Griesemer <[email protected]>

Author: griesemer

src/go/types/assignments.go

@@ -25,7 +25,9 @@ func (check *Checker) assignment(x *operand, T Type, context string) {
 	case constant_, variable, mapindex, value, commaok, commaerr:
 		// ok
 	default:
-		unreachable()
+		// we may get here because of other problems (issue #39634, crash 12)
+		check.errorf(x.pos(), "cannot assign %s to %s in %s", x, T, context)
+		return
 	}
 
 	if isUntyped(x.typ) {

src/go/types/fixedbugs/issue39634.go2

@@ -30,6 +30,9 @@ type foo8(type A) interface { type A }
 func bar8(type A foo8(A))(a A) {}
 func main8() {}
 
+// crash 12
+var u /* ERROR cycle */ , i [func /* ERROR used as value */ /* ERROR used as value */ (u, c /* ERROR undeclared */ /* ERROR undeclared */ ) {}(0, len)]c /* ERROR undeclared */ /* ERROR undeclared */
+
 // crash 15
 func y15() { var a /* ERROR declared but not used */ interface{ p() } = G15(string){} }
 type G15(type X) s /* ERROR undeclared name */