Unmarshal() does not use the env variable if the config file has no that key

[zzycn003]

Preflight Checklist

• I have searched the issue tracker for an issue that matches the one I want to file, without success.
• I am not looking for support or already pursued the available support channels without success.

Viper Version

1.9.0

Go Version

1.16

Config Source

Environment variables, Files

Format

YAML

Repl.it link

No response

Code reproducing the issue

type Redis struct {
   Addrs string `mapstructure:"addrs"`
   PoolSize int `mapstructure:"poolSize"`
}

var (
	cfgFile string
	cfg     Redis
)

// the config file as below
/**
{
  "addrs": "127.0.0.1:6379"
  // NOTE: the config file does not contains the key for poolSize, poolSize is injected from env
}
*/

/**
  env is POOLSIZE=100
*/

func main() {
       
	viper.SetConfigFile(cfgFile)
	viper.AutomaticEnv()
	viper.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))

	if err := viper.ReadInConfig(); err != nil {
		return fmt.Errorf("failed to load config file %s: %v", cfgFile, err)
	}
	if err := viper.Unmarshal(&cfg); err != nil {
		return fmt.Errorf("failed to marshal config file %s: %v", cfgFile, err)
	}
}

Expected Behavior

cfg.PoolSize == 100

Actual Behavior

cfg.PoolSize == 0

Steps To Reproduce

1. with config file, has no a key for poolSize
2. the poolSize is injected with environment
3. call viperh.Unmarshal

NOTE: only when the poolSize is defined in config file, the env override works

Additional Information

No response

[github-actions]

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[vorlif]

[x] I have searched the issue tracker for an issue that matches the one I want to file, without success.

I have the same problem and it is similar to that in

#188 #1012 #935 #761 #688 #464

Since the issue has existed since 2016, we probably have to hope that it will be resolved in Viper v2.

[sagikazarmark]

Please see #761 for a detailed analysis and potential workarounds. Closing as duplicate.