go - 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, e.g.

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 (i.e. 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 CustomTimes 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.