unexported-return: fix type aliases support (#1408)

rule/unexported_return.go

@@ -78,7 +78,18 @@ func (*UnexportedReturnRule) Name() string {
 // It is imprecise, and will err on the side of returning true,
 // such as for composite types.
 func exportedType(typ types.Type) bool {
-	switch t := types.Unalias(typ).(type) {
+	switch t := typ.(type) {
+	case *types.Alias:
+		obj := t.Obj()
+		switch {
+		// Builtin types have no package.
+		case obj.Pkg() == nil:
+		case obj.Exported():
+		default:
+			_, ok := t.Underlying().(*types.Interface)
+			return ok
+		}
+		return true
 	case *types.Named:
 		obj := t.Obj()
 		switch {

testdata/golint/unexported_return.go

@@ -58,3 +58,31 @@ type int struct{}
 func ExportedIntReturner() int { // MATCH /exported func ExportedIntReturner returns unexported type foo.int, which can be annoying to use/
 	return int{}
 }
+
+type config struct {
+	N int
+}
+
+// Option ...
+type Option = option
+
+type option func(*config)
+
+// WithN ...
+func WithN(n int) Option {
+	return func(c *config) {
+		c.N = n
+	}
+}
+
+type b = A
+
+// A ...
+type A func(*config)
+
+// WithA ...
+func WithA(n int) b { // MATCH /exported func WithA returns unexported type foo.b, which can be annoying to use/
+	return func(c *config) {
+		c.N = n
+	}
+}