User defined string types don't unmarshal

[dhiltgen]

A common pattern for mapping enums into config files is to define a new string type and consts for the acceptable values. This marshals correctly, but wont unmarshal properly.

Example:

package main

import (
        "fmt"
        "github.com/naoina/toml"
)

type Color string

const (
        Red  Color = "red"
        Blue Color = "blue"
)

type cfg struct {
        MyColor Color
}

func main() { 
        c1 := cfg{Red}

        data, err := toml.Marshal(&c1)
        if err != nil {
                panic(err)
        }
        fmt.Printf("Serialized: %s", string(data))

        var c2 cfg

        if err := toml.Unmarshal(data, &c2); err != nil {
                panic(err)
        }
        fmt.Printf("Unmarshaled: %#v\n", c2)
}

Output:

Serialized: my_color="red"
panic: toml: unmarshal: line 1: main.cfg.MyColor: `string' type is not assignable to `main.Color' type

goroutine 1 [running]:
panic(0x4d5260, 0xc42000a790)
    /usr/local/go/src/runtime/panic.go:500 +0x1a1
main.main()
    /home/daniel/code/go/toml/tagorder.go:31 +0x2a4
exit status 2

The same pattern works properly with https://golang.org/pkg/encoding/json/