Why Do Type Assertions Fail When Casting to Type Aliases in Go?

Casting to Type Aliases in Go

When casting to a type alias in Go, type assertions are used instead of casting. However, the behavior of type assertions differs from that of type conversions.

Type Conversions

Type conversions occur automatically when a value is stored in a variable of a different type. For example, assigning an int to a float64 variable results in a type conversion.

Type Assertions

Type assertions explicitly check if a value stored in an interface{} variable is of a specific type. They take the form a.(T), where a is the variable and T is the target type.

Issue with Casting to Type Aliases

As shown in the provided playground snippet, attempting to cast an interface{} variable to a type alias (somethingFuncy) results in a panic. This is because type assertions require the dynamic type of the value to be identical to the target type.

Explanation

In Go, type identity refers to two named types having the same type name and originating from the same TypeSpec. Since type aliases are effectively named types, they must have the exact same type name and declaration to be considered identical.

Conclusion

Casting to type aliases in Go using type assertions is not possible because the aliased type must be identical to the target type. Only the exact same named type can be asserted.

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