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go/types: unable to implement method of an interface with a generic return value #50828

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teken opened this issue Jan 26, 2022 · 2 comments
Closed

go/types: unable to implement method of an interface with a generic return value #50828

teken opened this issue Jan 26, 2022 · 2 comments
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FrozenDueToAge generics Issue is related to generics NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one.
Milestone
Go1.18

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@teken
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teken commented Jan 26, 2022

What version of Go are you using (go version)?

devel go1.18-6eb58cdffa Tue Jan 25 22:56:45 2022 +0000

Does this issue reproduce with the latest release?

Yes latest 1.18 beta release

What operating system and processor architecture are you using (go env)?

I am using go.dev/play

What did you do?

Go Playground

TL;DR I am trying to implement a method from an interface that has a generic to define it return type

I was trying to create a SumOf function that allowed for the selecting of a property for summing. I had a Product struct with a Cost property, I also have a SumCostProduct struct, that extends Product, that implements a Selectable interface that uses a Number constraint. This should allow for the implementation of the Selected function from the Selectable interface for SumCostProduct.

What did you expect to see?

I expected to compile and run the program with the return 13

What did you see instead?

./prog.go:26:14: cannot use generic type Selectable[E Number] without instantiation
./prog.go:29:14: v.Selected undefined (type T has no field or method Selected)

Go build failed.
@mknyszek mknyszek changed the title affected/package: Unable to implement function from a interface with a return of a constrained generic go/types: unable to implement method of an interface with a generic return value Jan 26, 2022
@mknyszek mknyszek added generics Issue is related to generics NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. labels Jan 26, 2022
@mknyszek mknyszek added this to the Go1.18 milestone Jan 26, 2022
@fzipp
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fzipp commented Jan 26, 2022

The correct function signature is:

func SumOf[T Selectable[TR], TR Number](items []T) TR {

(You forgot a [TR] after Selectable.)

https://go.dev/play/p/t-9qdtJQDI5?v=gotip

@mknyszek
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What you wrote isn't going to quite work because the fact is int never actually gets propagated to TR.

Here's a modified version of the code you wrote that works, and I think maintains the spirit of what you wanted: https://go.dev/play/p/N0ws6b0hov9?v=gotip

There may be a way to make Product non-generic, but I'm not really sure how to keep Selectable generic without it. It almost sounds like in this case Selectable should not be generic.

Anyway, as far as I can tell this is WAI, so not a Go issue. In the future, please ask questions first at https://groups.google.com/g/golang-nuts. Closing...

@golang golang locked and limited conversation to collaborators Jan 26, 2023
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FrozenDueToAge generics Issue is related to generics NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one.
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