Why Can't I Use Methods on a Pointer to a Type Parameter in Go Generics?

Generics in Go: Exploring Type Parameters and Constraints

Understanding Type Parameters and Constraints

In Go's generic programming model, type parameters represent placeholder types that can be instantiated with concrete types. However, type parameters do not imply any specific properties or method sets for their pointer types (*T). Constraints, on the other hand, define the requirements that concrete types must meet to implement a type parameter.

The Pointer to Type Parameter Issue

The error "type T is pointer to type parameter, not type parameter" arises when you attempt to use methods defined on a concrete type's pointer (T) as part of the type parameter's method set. This is because the type parameter and its pointer type are distinct entities.

Resolving the Issue with Constraints

To resolve this issue, you can explicitly declare constraints that require concrete types to implement methods on both the type and its pointer:

func Foo[T any, PT interface { SetId(string); *T}](v T) {}

Implementing Constraints

When implementing constraints, ensure that the methods are declared appropriately. In the case of the GS interface with a SetId method declared on T, you should instantiate MyStore with A and modify the type of the field in MyStore to match the concrete type:

var storeA = &MyStore[*A]{}
type MyStore[T GS] struct {
    values map[string]T 
}
func (s *MyStore[T]) add(item T) {}

By refining the constraints and implementing methods accordingly, you can utilize generics effectively in your Go code.

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