Constrain a type parameter to be assignable to another

[Michael Andersen]

Hi

Is there a way to constrain one parameter to be assignable to another?

For example, in ye olde days, I'd have:

type Iface interface {
// ...
}

type Concrete1 struct { /* ... */ }
var _ Iface = &Concrete1{}

type Concrete2 struct{ /* ... */ }
var _ Iface = &Concrete2{}

// Repeat ad nauseam

Now, I'm finding myself wanting similar compile time checks for generic types and functions. E.g.

func foo[Iface any, Concrete Iface]() {
  var c Concrete
  var _ Iface = c
}

Obviously that's a bit contrived, but I'm writing code that needs both the concrete type (*someStruct) and the interface it's meant to adhere to.

The above code does not compile with 1.18 (cannot use a type parameter as constraint)

I have a runtime check that works, but I'd love a compile time check. Is there a way to do that?

Thanks

Michael

[Axel Wagner]

No, it's not possible. If there is a compelling enough use-case, it might be made possible in the future (though not sure, how), but as it stands, it isn't.

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[Ian Lance Taylor]

On Wed, Mar 16, 2022 at 9:44 PM Michael Andersen <mic...@steelcode.com> wrote:
>
> Is there a way to constrain one parameter to be assignable to another?

Currently there is not. We're not sure how to handle that case. See
https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#no-way-to-express-convertibility
.

Ian