You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: authhandler.go
    path of file: ./repos/teller/vendor/golang.org/x/oauth2/authhandler
    
    the code of the file until where you have to start completion: // Copyright 2021 The Go Authors. All rights reserved.
// Use of this source code is governed b
    
func (source authHandlerSource) Token() (*oauth2.Token, error) {
	// Step 1: Obtain auth code.
	var authCodeUrlOptions []oauth2.AuthCodeOption
	if source.pkce != nil && source.pkce.Challenge != "" && source.pkce.ChallengeMethod != "" {
		authCodeUrlOptions = []oauth2.AuthCodeOption{oauth2.SetAuthURLParam(codeChallengeKey, source.pkce.Challenge),
			oauth2.SetAuthURLParam(codeChallengeMethodKey, source.pkce.ChallengeMethod)}
	}
	url := source.config.AuthCodeURL(source.state, authCodeUrlOptions...)
	code, state, err := source.authHandler(url)
	if err != nil {
		return nil, err
	}
	if state != source.state {
		return nil, errors.New("state mismatch in 3-legged-OAuth flow")
	}

	// Step 2: Exchange auth code for access token.
	var exchangeOptions []oauth2.AuthCodeOption
	if source.pkce != nil && source.pkce.Verifier != "" {
		exchangeOptions = []oauth2.AuthCodeOption{oauth2.SetAuthURLParam(codeVerifierKey, source.pkce.Verifier)}
	}
	return source.config.Exchange(source.ctx, code, exchangeOptions...)
}