How can I instantiate a non-nil pointer of type argument with generic Go?

Question

Basically you have to add one more type parameter to the constraint to make T convertible to its pointer type. In its most basic form, this technique looks like the following (with an anonymous constraint):

func Foo[T any, PT interface { *T; M() }]() {
    p := PT(new(T))
    p.M() // calling method on non-nil pointer
}

Playground: https://go.dev/play/p/L00tePwrDfx

Step by step solution

Your constraint SetGetter already declares a type param V, so we slightly modify the example above:

// V is your original type param
// T is the additional helper param
type SetGetter[V any, T any] interface {
    Set(V)
    Get() V
    *T
}

Then you define the SetGetterSlice function with the type parameter T any, whose purpose is just to instantiate the constraint SetGetter.

You will then be able to convert the expression &out[i] to the pointer type, and successfully call the method on the pointer receiver:

// T is the type with methods with pointer receiver
// PT is the SetGetter constraint with *T
func SetGetterSlice[V any, T any, PT SetGetter[V, T]](values []V) []T {
    out := make([]T, len(values))

    for i, v := range values {
        // out[i] has type T
        // &out[i] has type *T
        // PT constraint includes *T
        p := PT(&out[i]) // valid conversion!
        p.Set(v)         // calling with non-nil pointer receiver
    }

    return out
}

Full program:

package main

import (
    "fmt"
)

type SetGetter[V any, T any] interface {
    Set(V)
    Get() V
    *T
}

func SetGetterSlice[V any, T any, PT SetGetter[V, T]](values []V) []T {
    out := make([]T, len(values))

    for i, v := range values {
        p := PT(&out[i])
        p.Set(v)
    }

    return out
}

// Count implements SetGetter interface
type Count struct {
    x int
}

func (c *Count) Set(x int) { c.x = x }
func (c *Count) Get() int  { return c.x }

func main() {
    ints := []int{1, 2, 3, 4, 5}

    // instantiate with base type
    sgs := SetGetterSlice[int, Count](ints)

    for _, s := range sgs {
        fmt.Println(s.Get()) // prints 1,2,3,4,5 each in a newline
    }
}

This becomes more verbose because SetGetterSlice now requires three type parameters: the original V plus T (the type with pointer receivers) and PT (the new constraint). However when you call the function, you can omit the third one – with type inference, both type params V and T required to instantiate PT SetGetter[V,T] are already known:

SetGetterSlice[int, Count](ints)

Playground: https://go.dev/play/p/gcQZnw07Wp3

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