How to Safely Convert a Generic Type Pointer to a Specific Interface in Go?

Cannot Convert Generic Type Parameter to Specific Type

In Go, when using generics, it's important to understand the relationship between generic type parameters and their constraints. In this case, the problem stems from trying to pass a pointer to a generic type parameter (*T) as an argument to a function that expects a specific interface (stringer).

The error message explains that *T is a pointer to a type parameter, but it doesn't implement the stringer interface. This is because the constraints of T (FooBar) and the stringer interface are not inherently connected. To resolve this, it's necessary to establish a relationship between the two.

Solution 1: Asserting Type Safety (Unsafe)

One solution is to assert that *T is a stringer using any(). This allows you to pass the pointer as an argument, but it sacrifices type safety and could lead to runtime panics.

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

Solution 2: Parameterizing FooBar (Type Safe)

Another solution is to parameterize the FooBar interface with the generic type T, ensuring that the type parameter of FooBar and the type parameter you pass to blah match. This approach preserves type safety and allows you to pass pointers to specific types that implement the stringer interface.

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

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

Explanation:

• The constraints of T are now foo or bar, and FooBar is parameterized with T.
• You can create a pointer to a specific type (e.g., *foo) and pass it to blah, which will instantiate FooBar with the corresponding type.
• The conversion U(&t) is now valid because U and the pointer to T have the same type set.

This solution enables you to pass instances of specific types that implement the stringer interface and ensures type safety during instantiation.

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