Methods unavailable on concrete type when used to instantiate generic in receiver

[Vinicius Hespanha Souza]

Good evening.

I observed this weird behavior today and wanted to see if that's somehow intended or if I found a bug. Couldn't find any other mention of it in the list but I guess there are a lot of ways to describe it.

I think the paste is pretty self-explanatory, but basically I can't call a concrete type's method if i'm inside a receiver that uses an instantiated generic with a type present in a constraint union.

paste.sr.ht/~vhespanha/58541b8628ca7dc2509ea85fb05f83900f3295e1

Thanks in advance if anyone can clear that up for me.

[Sean Liao]

func (h *holder[alpha]) processAlpha()

this doesn't constrain the type to alpha, you've just renamed the type constraint of your holder type to shadow the alpha type declaration.

- sean

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[Brian Candler]

I came across this too:

https://groups.google.com/g/golang-nuts/c/YJiS4eGLVdk/m/dPyqd0EMAQAJ

You can think of the declaration as being like this:

func (h *holder[alpha any]) processAlpha()

(although this is not valid syntax today). The name "alpha" gets bound to some type parameter in local scope, so it is unrelated to the "alpha" which exists outside. You can even shadow names like "string" and "int" in Go, if you want to write really confusing code.

I'm not sure if what you were trying to do is something like this:

type T = *holder[alpha]

func (h T) processAlpha() {
item := alpha(0.1)
_ = item.validate()
}

If so, that's not allowed:

./prog.go:28:9: cannot define new methods on instantiated type holder[alpha] 

[Ian Lance Taylor]

I'm not sure but perhaps it will help to read
https://go.dev/doc/faq#types_in_method_declaration .

Ian