Getting the base type of a custom type using Reflect

Question

Say I create a custom type in Go:

type CustomTime time.Time

Using reflection, I'm comparing types, eg

var foo CustomTime = CustomTime(time.Now())
customType := reflect.TypeOf(foo)

timeType := reflect.TypeOf(time.Now())

if customType == timeType {
    fmt.Println("Is timeType")
} else {
    fmt.Println("Is not timeType")
}

This prints "Is not timeType". What I'm trying to do is find a way to see if the base type that the custom type uses (ie the time.Time in type CustomType time.Time ) is of type time.Time .

I've tried using reflect's Kind() function, but that returns struct for both since time.Time is a struct.

Here's a playground with this code.

Answer 1

You can't exactly, because that's not how types work in Go. In your example, CustomTime 's underlying type isn't Time ; both CustomTime and time.Time share the same underlying type, which is a struct type. From the docs :

Each type T has an underlying type: If T is one of the predeclared boolean, numeric, or string types, or a type literal, the corresponding underlying type is T itself. Otherwise, T's underlying type is the underlying type of the type to which T refers in its type declaration.

That means that CustomTime s underlying type isn't time.Time , it's time.Time 's underlying type .

You can use reflect.Type 's ConvertibleTo() method to see if one type is convertible to another, which is as close as you're likely to get to what you're describing.