Why Does Go's Type Assertion Fail with Type Aliases?

How to Cast to a Type Alias in Go?

Consider the following Go code:

package main

import "fmt"

type somethingFuncy func(int) bool

func funcy(i int) bool {
    return i%2 == 0
}

func main() {
    var a interface{} = funcy

    _ = a.(func(int) bool)  // Works

    fmt.Println("Awesome -- apparently, literally specifying the func signature works.")

    _ = a.(somethingFuncy)  // Panics

    fmt.Println("Darn -- doesn't get here. But somethingFuncy is the same signature as func(int) bool.")
}

The first type assertion works when explicitly declaring the type as func(int) bool. However, the second one using the type alias somethingFuncy panics.

Explanation

Unlike casting, type assertions in Go strictly compare the actual type of the value being asserted. Therefore, the type alias somethingFuncy, although sharing the same signature as func(int) bool, is considered a distinct type.

Bonus

The type assertion is performed in the following code, which simply compares the types for equality:

func main() {
    var a interface{} = funcy

    switch v := a.(type) {
    case func(int) bool:
        // Type successfully asserted as func(int) bool
    case somethingFuncy:
        // Type successfully asserted as somethingFuncy
    default:
        // Type assertion failed
    }
}

This comparison fails when using the type alias because it requires an exact type match.

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