cmd/compile: misleading error message when using type parameter in typeset

[icholy]

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

devel go1.18-c8861432b8 Sun Jan 2 14:27:43 2022 +0000

What did you do?

type X[T any] interface {
	T | string
}

https://go.dev/play/p/v-8zaOs_d3r?v=gotip

What did you expect to see?

"cannot use type parameter in typeset"

What did you see instead?

"cannot embed a type parameter"

[zigo101]

This is specified in https://tip.golang.org/ref/spec#Interface_types

In a union, a term cannot be a type parameter ...

[edit], Aha, you mean the message is not clear. Okay, it is some of.

[thanm]

@griesemer per owners.

Personally I think "cannot use type parameter in typeset" is perfectly clear as it is.

[griesemer]

Just to be clear (@thanm ), the current error message is: cannot embed a type parameter.
cc: @findleyr for suggestions.

[findleyr]

I'm not sure if the word(s) 'type set' (or typeset) are appropriate here, since it refers to the entire type set of the interface type. Maybe "cannot use a type parameter in a union", which is the phrasing currently used in the spec.

[findleyr]

...though it may also be confusing that *T is valid here, as in

type X[T any] interface {
	*T | string
}

Maybe union terms cannot be type parameters is sufficiently specific.

[zigo101]

Some off-topic, should "union" be replaced with "intersection"? int | string means allowing the operations which are legal for both int and string, instead of for either int or string.

[ianlancetaylor]

int | string means that both int and string are in the type set of the interface. More generally T1 | T2 means that the union of the type sets of T1 and T2 are in the type set of the interface. So union is the correct term.

It's true that because an operation is supported if it is supported by all types in the type set, one can view operations as going by the intersection. But from a type set perspective it is definitely a union operator.

See #45346.

[gopherbot]

Change https://go.dev/cl/396874 mentions this issue: go/types, types2: better error message for invalid type parameter term

[gopherbot]

Change https://go.dev/cl/396915 mentions this issue: spec: clarify type term restriction for type parameters