encoding/json doesn't find marshal/unmarshal via type aliases

[Marcin Romaszewicz]

Hi All,

I've created a simple struct that wraps time.Time because I'd like to unmarshal it differently from JSON than the default. It works great.

However, when I pass a type alias of that struct to json.Marshal or json.Unmarshal, the MarshalJSON and UnmarshalJSON functions aren't invoked, despite both types conforming to the interfaces.

Here's the shortest example I could make.

https://go.dev/play/p/pETWQB3CWxV

This is where this came up:
https://github.com/deepmap/oapi-codegen/issues/579

I did find a workaround, in redeclaring the MarshalJSON/UnmarshalJSON on the aliased type, but I don't understand why this is necessary, and I'm hoping someone who understands the innards of this better than I do could enlighten me.

Thanks,

-- Marcin

[Axel Wagner]

It's not an "aliased type". Type aliases take the form

type A = B

and they make the name A be interchangeable with the name B. What you have is a type declaration

type A B

which creates a fully new type, with a new method set, but the same underlying type as B.

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[Marcin Romaszewicz]

Sorry about mixing up terms, however, my question still stands.

encoding/json looks for the Marshaler/Unmarshaler interface convertibility, and in this case, the compiler thinks that the redeclared type does implement json.Unmarshaler. I extended my example to also do this test via reflection.

https://go.dev/play/p/TrxM2zxG2pX

So, yes, it's a new type, but it seems to behave just like the base type.

[Sean Liao]

As explained above, `type AliasedDate Date` clears any methods you have defined on `Date`.

But `Date` embeds `time.Time` which has its own `UnmrshalJSON`,

which now gets promoted.

So `Date.UnmsrshalJSON` calls your `UnmsrshalJSON`

`AliastedDate.UnmsrshalJSON` calls `AliastedDate.Time.UnmsrshalJSON`

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[Marcin Romaszewicz]

Aha! That is what I was missing. Thank you.

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