unused: don't ignore fields and methods of aliased type in imported package

When ignoring an alias, we don't want to ignore the fields and methods
of the aliased type if that type is in a different package. Not only
is it unnecessary, it also breaks the invariant that we only use
objects we've already seen.

Closes gh-1073

(cherry picked from commit 6c9d8ed)

Author: dominikh

unused/testdata/src/alias/alias.go

@@ -1,5 +1,7 @@
 package main
 
+import "net/http"
+
 type t1 struct{} // used
 type t2 struct{} // unused
 type t3 struct{} // used
@@ -13,3 +15,20 @@ func main() { // used
 	var _ alias1
 	var _ t3
 }
+
+type t4 struct { // used
+	x int // used
+}
+
+func (t4) foo() {} // used
+
+//lint:ignore U1000 alias5 is ignored, which also ignores t4
+type alias5 = t4 // used
+
+//lint:ignore U1000 alias6 is ignored, and we don't incorrectly try to include http.Server's fields and methods in the graph
+type alias6 = http.Server // used
+
+//lint:ignore U1000 aliases don't have to be to named types
+type alias7 = struct { // used
+	x int // used
+}

unused/unused.go

@@ -1206,6 +1206,17 @@ func (g *graph) entry(pkg *pkg) {
 
 					// use methods and fields of ignored types
 					if obj, ok := obj.(*types.TypeName); ok {
+						if obj.IsAlias() {
+							if typ, ok := obj.Type().(*types.Named); ok && typ.Obj().Pkg() != obj.Pkg() {
+								// This is an alias of a named type in another package.
+								// Don't walk its fields or methods; we don't have to,
+								// and it breaks an assertion in graph.use because we're using an object that we haven't seen before.
+								//
+								// For aliases to types in the same package, we do want to ignore the fields and methods,
+								// because ignoring the alias should ignore the aliased type.
+								continue
+							}
+						}
 						if typ, ok := obj.Type().(*types.Named); ok {
 							for i := 0; i < typ.NumMethods(); i++ {
 								g.use(typ.Method(i), nil, edgeIgnored)