generic-type-t-any-cannot-be-nil.md

title	tagline	description
Generic type `T any` cannot be `nil`	Golang code with unexpected behaviors	Unexpected golang behavior when trying to return a `nil` value for a generic type `T any`

Generic type T any cannot be nil

The generic type T any is a new feature in go1.18 that allows you to define a function that can accept any type, and return any type.

However, you can't return nil for a generic type T any, for example, this code would be invalid:

func doSomething[T any]() T {
    return nil
}

Error: cannot use nil as T value in return statement

The compiler will return an error like this:

cannot use nil as T value in return statement

At first glance, one might think it's unexpected, however notice that the generated code should be valid for specific types. The type T isn't really an interface{} type (any), but a placeholder for a specific type.

So if the compiler wanted to build the doSomething function for a string, it would be like this:

func doSomething() string {
    return nil
}

Which is now obviously invalid.

How to return zero value for a generic type T any

If you want to return the zero value for a generic type T any, you can just declare a variable of type T and return it:

package main

import "fmt"

func nothing[T any]() T {
	var zero T
	return zero
}

func main() {
	fmt.Printf("string: %q\n", nothing[string]())
	fmt.Printf("*int: %v\n", nothing[*int]())
}

If you run this example in go.dev playground, it would print:

string: ""
*int: <nil>