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spec: document that type parameters are not permitted as constant declaration types #50202

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gazerro opened this issue Dec 15, 2021 · 4 comments
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@gazerro
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@gazerro gazerro commented Dec 15, 2021

The compilation of the following program fails with error ./prog.go:4:10: invalid constant type P

package main

func f[P int]() {
	const y P = 5
}

func main() {}

but this behavior is not documented in the spec.

From the spec 5 is assignable to P because it is an untyped constant representable by a value of type P.

@ianlancetaylor
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@ianlancetaylor ianlancetaylor commented Dec 16, 2021

@ianlancetaylor ianlancetaylor added the NeedsInvestigation label Dec 16, 2021
@ianlancetaylor ianlancetaylor added this to the Go1.18 milestone Dec 16, 2021
@griesemer
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@griesemer griesemer commented Dec 16, 2021

I believe this is not yet fully documented in the in-progress spec. But here it is: type parameters cannot be constant types. While 5 can be stored in a variable of type P, it cannot be the value of a constant of type P. Specifically, and this is documented in the section on conversions, if a constant is converted to a type parameter, say P(5), the result is a non-constant value.

Leaving open until fully documented.

@griesemer griesemer self-assigned this Dec 16, 2021
@griesemer griesemer removed the NeedsInvestigation label Dec 16, 2021
@gazerro
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@gazerro gazerro commented Dec 16, 2021

@gazerro Are you looking at https://tip.golang.org/ref/spec?

Yes.

@gopherbot
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@gopherbot gopherbot commented Feb 9, 2022

Change https://go.dev/cl/384240 mentions this issue: spec: the type of a constant cannot be a type parameter

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