Why Doesn't My Go Generic Type Parameter Satisfy the `stringer` Interface?

Understanding Generic Type Constraints in Go

In Go 1.18, generics offer a powerful tool for enhancing code flexibility. However, when working with type constraints, it's important to understand why certain errors can occur.

The Problem

Consider the following code:

type stringer interface {
    a() string
}

func do(s stringer) {
    fmt.Println(s.a())
}

func blah[T FooBar]() {
    t := &T{}
    do(t)
}

func main() {
    blah[foo]()
}

When attempting to compile this code, you may encounter the error:

cannot use t (variable of type *T) as type stringer in argument to do: *T does not implement stringer (type *T is pointer to type parameter, not type parameter)

Understanding the Error

This error stems from a misunderstanding of how generic type constraints work. The constraint FooBar in the blah function is a placeholder for types that satisfy the interface FooBar. However, the variable t is of type T, which is a pointer to the type parameter T. This means that T is not a type parameter itself, and therefore it cannot satisfy the stringer interface.

The Solution

The solution to this issue is to introduce a relationship between the type parameter T and the stringer interface. There are two potential approaches:

1. Explicit Assertion

You can explicitly assert that *T satisfies the stringer interface using the any type conversion:

func blah[T FooBar]() {
    t := &T{}
    do(any(t).(stringer))
}

2. Type Composition

Alternatively, you can define a new type that combines the constraints of FooBar and stringer:

type FooBar[T foo | bar] interface {
    *T
    stringer
}

func blah[T foo | bar, U FooBar[T]]() {
    var t T
    do(U(&t))
}

This approach ensures type safety by embedding stringer into the FooBar interface and requires that T be a pointer type to satisfy the constraint FooBar.

Conclusion

Understanding the relationship between type constraints and type parameters is crucial for effectively working with generics in Go. By introducing a relationship between T and stringer, either through explicit assertion or type composition, you can resolve the error and enable the desired behavior.

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