This may be a duplicate of #50259 but in the example in that bug report, the types are incorrect and should not compile, whereas I believe the types in this report are correct and should compile.

This is blocking development.

Just curious as to why the alias is required? When I take it out (e.g. replace line 21 with "type Step Pair[Box, interface{}]" your code seems to compile ok.

Aliases are a pretty boutique/obscure Go feature -- they are (in my experience) really only needed when moving types around between packages.

This is blocking development.

Just curious as to why the alias is required? When I take it out (e.g. replace line 21 with "type Step Pair[Box, interface{}]" your code seems to compile ok.

That's a fair point, this is work-aroundable in that way.

Aliases are a pretty boutique/obscure Go feature -- they are (in my experience) really only needed when moving types around between packages.

I suspect that you'll find aliases come into heavy use now that Go has generic types -- it's suddenly an important form of abstraction. You're right that I can replace every instance of Step with Pair[Box, interface{}] but that is generally burdensome if I'm using Step in a lot of places, and in particular makes a refactoring (say, to replace interface{} with a more specific type) more painful than it ought to be.

You're right that I can replace every instance of Step with Pair[Box, interface{}]

Just to be clear, I never made that suggestion. I just suggested that you use

type Step Pair[Box, interface{}]

instead of

type Step = Pair[Box, interface{}]

Thanks.

You're right that I can replace every instance of Step with Pair[Box, interface{}]

Just to be clear, I never made that suggestion. I just suggested that you use

type Step Pair[Box, interface{}]

instead of

type Step = Pair[Box, interface{}]

Thanks.

Ugh, indeed you did. Total failure of reading comprehension on my part. My apologies. That also works, including in the original codebase. It feels slightly dirty as it creates a new type where I don't really intend to, but the semantics works out in the end.

Simpler reproducer:

package p

type Transform[T any] struct{}
type Pair[S any] struct {}

// Comment this line out to make this compile
var first Transform[Step]

type Box Transform[Step]
type Step = Pair[Box]

The type checker passes this code (though I have not yet verified that it's doing the correct thing).

I agree that this is likely a duplicate of #50259. Leaving open for now until confirmed.

Removing release-blocker label as there are work-arounds for this code.

Confirming that this is a duplicate of #50259. Will be closed with forthcoming fix.

Change https://golang.org/cl/379916 mentions this issue: cmd/compile/internal/types2: report an error when using a broken alias

Change https://golang.org/cl/380056 mentions this issue: cmd/compile/internal/types2: reorder object processing to avoid broken aliases