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spec: constraint type inference for struct fields #57270

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iangudger opened this issue Dec 12, 2022 · 1 comment
Closed

spec: constraint type inference for struct fields #57270

iangudger opened this issue Dec 12, 2022 · 1 comment
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FrozenDueToAge generics Issue is related to generics NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. TypeInference Issue is related to generic type inference
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@iangudger
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What version of Go are you using (go version)?

$ go version
go version go1.19.4 linux/amd64

Does this issue reproduce with the latest release?

Yes

What did you do?

type Element[T any] interface {
	*T
}

type Entry[T any, U Element[T]] struct{}

type testEntry struct {
	Entry[testEntry]
}

https://go.dev/play/p/RbzMxIySi3f

What did you expect to see?

Compiles.

What did you see instead?

./prog.go:10:2: got 1 arguments but 2 type parameters

If we modify the code above to the following, it compiles:

type Element[T any] interface {
	*T
}

type Entry[T any, U Element[T]] struct{}

type testEntry struct {
	Entry[testEntry, *testEntry]
}

https://go.dev/play/p/taePA8Kizz1
@ianlancetaylor ianlancetaylor changed the title types2: constraint type inference for struct fields spec: constraint type inference for struct fields Dec 13, 2022
@ianlancetaylor ianlancetaylor added NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. generics Issue is related to generics TypeInference Issue is related to generic type inference labels Dec 13, 2022
@ianlancetaylor ianlancetaylor added this to the Backlog milestone Dec 13, 2022
@griesemer
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The issue here is not missing inference for struct fields but missing type inference for generic types (Entry is a generic type). If Entry is a generic function, type inference does work (adjusted example).

We don't do type inference for types because it is significantly more complex than type inference for functions: generic types may refer to themselves recursively in ways that may influence type inference. We don't have a good understanding what that means.

We are not planning to do this any time soon, if at all. In any case, it would require a thorough proposal. Closing this issue.

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FrozenDueToAge generics Issue is related to generics NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. TypeInference Issue is related to generic type inference
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