Make Viper.Sub() inherit values from the bound pflags of the parent

Sub creates a new instance from a sub-tree, but bound flags
are not passed down to the new instance. This commit adds
the bound flags that are prefixed as the provided k to the
new instance.

viper.go

@@ -780,11 +780,21 @@ func (v *Viper) Sub(key string) *Viper {
 		return nil
 	}
 
-	if reflect.TypeOf(data).Kind() == reflect.Map {
-		subv.config = cast.ToStringMap(data)
-		return subv
+	if !(reflect.TypeOf(data).Kind() == reflect.Map) {
+		return nil
 	}
-	return nil
+	subv.config = cast.ToStringMap(data)
+	subPFlags := make(map[string]FlagValue)
+	for flagName, flagValue := range v.pflags {
+		keyPrefix := key + "."
+		if !strings.HasPrefix(flagName, keyPrefix) {
+			continue
+		}
+		newFlagName := flagName[len(keyPrefix):]
+		subPFlags[newFlagName] = flagValue
+	}
+	subv.pflags = subPFlags
+	return subv
 }
 
 // GetString returns the value associated with the key as a string.

viper_test.go

@@ -1253,6 +1253,32 @@ func TestSub(t *testing.T) {
 	assert.Equal(t, (*Viper)(nil), subv)
 }
 
+func TestSubWithBoundedFlags(t *testing.T) {
+	v := New()
+	v.SetConfigType("yaml")
+	v.ReadConfig(bytes.NewBuffer(yamlExample))
+
+	overridenValue := "notSoLarge"
+	flags := pflag.NewFlagSet("test", pflag.ContinueOnError)
+	flags.String("clothing.pants.size", "", "")
+	assert.NoError(t, flags.Parse([]string{"--clothing.pants.size=" + overridenValue}))
+
+	v.BindPFlags(flags)
+	subv := v.Sub("clothing")
+	assert.Equal(t, overridenValue, v.Get("clothing.pants.size"))
+	assert.Equal(t, overridenValue, subv.Get("pants.size"))
+	assert.Equal(t, v.Get("clothing.pants.size"), subv.Get("pants.size"))
+
+	subv = v.Sub("clothing.pants")
+	assert.Equal(t, v.Get("clothing.pants.size"), subv.Get("size"))
+
+	subv = v.Sub("clothing.pants.size")
+	assert.Equal(t, (*Viper)(nil), subv)
+
+	subv = v.Sub("missing.key")
+	assert.Equal(t, (*Viper)(nil), subv)
+}
+
 var hclWriteExpected = []byte(`"foos" = {
   "foo" = {
     "key" = 1