Understanding the "Cannot Use Variable of Type *T as Type Stringer in Argument" Error in Go Generics

In the code snippet provided, you're attempting to generically invoke a function do that expects a parameter of type stringer, but you're passing a pointer to a type parameter T instead.

The Go compiler is raising an error because:

• Type Identity and Parameters: T is not inherently equivalent to its constraint FooBar. Therefore, *T is not the same as *FooBar or *bar.
• Missing Method: The stringer interface requires a method named a(). Currently, your foo and bar types implement this method on pointer receivers, but *T does not inherently possess this method.

Fixing the Issue

There are two main approaches to resolving this issue:

1. Assert Type Safety with Type Assertion (Less Optimal)

• Assert that *T implements stringer using any(t).(stringer) within the do function.
• This approach may lead to panics if T does not actually implement stringer.

2. Redefine Constraint and Customize Generics (Preferred)

• Add stringer as a constraint to the FooBar interface.
• Use pointer receivers for the foo and bar types to align with the constraint.
• Introduce a second type parameter that specifies the constraint within the FooBar interface.
• Pass an instance of the constrained type as an argument to blah.

Revised Code:

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

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

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

In this revised code:

• The FooBar interface includes stringer as a constraint.
• The foo and bar types have pointer receivers for their a() methods.
• U is constrained by FooBar[T], ensuring that it satisfies both FooBar and stringer constraints.
• blah takes an argument of type U, which is an instance of the constrained FooBar interface.