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go/types: Cannot return T from method receiving Generic[T] #57182

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fschuetz04 opened this issue Dec 8, 2022 · 5 comments
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go/types: Cannot return T from method receiving Generic[T] #57182

fschuetz04 opened this issue Dec 8, 2022 · 5 comments
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@fschuetz04
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What version of Go are you using (go version)?

Go Playground with Go 1.18, 1.19, and dev branch.

Does this issue reproduce with the latest release?

Yes.

What did you do?

See https://go.dev/play/p/fCO1GDA_oZj?v=gotip for an example.

I defined a generic type, e. g.:

type Generic[T any] struct{}

Then I wanted to add a method receiving an instantiation of this generic type and returning T, e. g.:

func (_ Generic[uint8]) Value() uint8 {
	return 0
}

What did you expect to see?

I would expect the program to build without errors.

What did you see instead?

Adding the method leads to an error when building the program:

cannot use 0 (untyped int constant) as uint8 value in return statement

The program only builds if the return type of the method is different from T.
@seankhliao
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this doesn't do what you think it does.
methods can't be on specific instantiations.

Unlike many projects, the Go project does not use GitHub Issues for general discussion or asking questions. GitHub Issues are used for tracking bugs and proposals only.

For questions please refer to https://github.com/golang/go/wiki/Questions

@seankhliao seankhliao closed this as not planned Won't fix, can't repro, duplicate, stale Dec 9, 2022
@timothy-king
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func (_ Generic[uint8]) Value() uint8 { return 0 } does not mean what you think it means. This is the same as func (_ Generic[T]) Value() T { return 0 }. You are overloading the name uint8 with a type parameter named "uint8" that is constrained to be an any. This is the meaning of the "uint8" return value. Hence the very confusing error message.

@timothy-king
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I vaguely recall there was a suggested vet check for this.

@seankhliao
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the check was deemed premature at the time #48123

@ianlancetaylor
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https://go.dev/doc/faq#types_in_method_declaration

@seankhliao seankhliao mentioned this issue Jun 23, 2023
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@timothy-king @ianlancetaylor @fschuetz04 @gopherbot @seankhliao