don't break types implementing interfaces

main.go

@@ -305,6 +305,10 @@ func transformGo(node ast.Node, info *types.Info) ast.Node {
 			case *types.Const:
 			case *types.TypeName:
 			case *types.Func:
+				sign := obj.Type().(*types.Signature)
+				if obj.Exported() && sign.Recv() != nil {
+					return true // might implement an interface
+				}
 				switch node.Name {
 				case "main", "init":
 					return true // don't break them

testdata/scripts/implement.txt

@@ -0,0 +1,28 @@
+garble build main.go
+exec ./main
+cmp stdout main.stdout
+
+! grep 'unexportedMethod' main
+
+-- main.go --
+package main
+
+import "fmt"
+
+type T string
+
+func (t T) String() string {
+	return "String method for " + string(t)
+}
+
+func (t T) unexportedMethod() string {
+	return "unexported method for " + string(t)
+}
+
+func main() {
+	fmt.Println(T("foo"))
+	fmt.Println(T("foo").unexportedMethod())
+}
+-- main.stdout --
+String method for foo
+unexported method for foo