find return types of specified name method

scratch_1.go

package main

import (
	"fmt"
	"go/ast"
	"go/parser"
)
import "golang.org/x/tools/go/loader"

func main() {
	source := `
package main

import "fmt"

type s struct{}

func (s s)Println(x string) {}


func increment(v int) int {
	return v + 1
}

func main(){
  fmt.Println("xxx")

  {
      fmt := s{}
      fmt.Println("yyy")
  }

  fmt.Println("xxx")
}
`

	lo := loader.Config{ParserMode: parser.ParseComments}
	astf, err := lo.ParseFile("main.go", source)
	if err != nil {
		panic(err)
	}
	lo.CreateFromFiles("main", astf)

	prog, err := lo.Load()
	if err != nil {
		panic(err)
	}
	targetPkgName := "fmt"
	targetFuncName := "Println"
	targetPkg := prog.Package(targetPkgName)
	fmt.Println(targetPkg)
	var targetSelectorExpr *ast.SelectorExpr
	for _, file := range targetPkg.Files {
		ast.FileExports(file)
		for _, decl := range file.Decls {
			if funcDecl, ok := decl.(*ast.FuncDecl); ok {
				if funcDecl.Name.Name == targetFuncName {
					results := funcDecl.Type.Results.List
					for _, result := range results {
						if typeIdent, ok := result.Type.(*ast.Ident); ok {
							fmt.Println(typeIdent.Name)
						}
					}
				}
			}
		}
	}

	_ = ast.Print(nil, targetSelectorExpr)
}