Why Does Go Throw '*T is pointer to type parameter, not type parameter' Error When Using Generics?

Go with Generics: Exploring "*T is pointer to type parameter, not type parameter"

In the realm of Go programming, when utilizing generics it's crucial to understand the nature of type parameters and their constraints. A common pitfall lies in confusing the type parameter itself with its constraints.

Consider the example of implementing an object store for types A and B with a shared ID field. Hoping to follow the DRY principle, a developer employs generics to create a store with a GS interface representing common operations. However, when attempting to add an object and set its ID field using the GS interface, the compiler raises an error:

item.SetId undefined (type *T is pointer to type parameter, not type parameter)

A does not implement GS (SetId method has pointer receiver)

Decoding this error message leads to a foundational understanding: a type parameter is distinct from its constraint. The constraint establishes the permitted operations on T but does not impose any requirements on T (the pointer to T). Consequently, the method set of T doesn't inherit the pointer receiver methods declared on the concrete type A, nor does it implicitly implement interfaces applicable to *A.

Resolving this issue involves explicitly defining additional constraints, as demonstrated in the following example:

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

To address the second part of the error, concerning the implementation of constraints, it's important to note that MyStore should be instantiated with A to satisfy the constraint that SetId() is defined on A, not A. Subsequently, adjusting the type of the struct field and method signature to reflect this change would allow for the desired behavior.

In summary, effectively handling "*T is pointer to type parameter, not type parameter" error requires a clear distinction between type parameters and their constraints. This ensures that constraints are explicitly declared and implemented, preventing unexpected compiler errors.

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