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mdempsky opened this issue Oct 5, 2022 · 2 comments
Closed

reflect: Implements fails with anonymous, recursive interfaces #56063

mdempsky opened this issue Oct 5, 2022 · 2 comments
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compiler/runtime Issues related to the Go compiler and/or runtime. FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done.
Milestone
Unplanned

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@mdempsky
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mdempsky commented Oct 5, 2022

Program below should print and exit silently, but instead it reports an error.

The underlying issue here may be a cmd/compile issue though, since it works with go run -compiler=gccgo.

$ go run q.go
FAIL: "main.T".Implements("main.I") reports false

$ cat q.go
package main

import (
	"fmt"
	"reflect"
)

func main() {
	// statically verify that T implements I
	var _ I = T(0)

	i := reflect.TypeOf(new(I)).Elem()
	t := reflect.TypeOf(new(T)).Elem()
	if !t.Implements(i) {
		fmt.Printf("FAIL: %q.Implements(%q) reports false\n", t, i)
	}
}

type I interface{ m() interface{ I } }

type T int

func (T) m() interface{ I } { return nil }

/cc @golang/compiler
@mdempsky mdempsky added the NeedsFix The path to resolution is known, but the work has not been done. label Oct 5, 2022
@mdempsky mdempsky added this to the Unplanned milestone Oct 5, 2022
@gopherbot gopherbot added the compiler/runtime Issues related to the Go compiler and/or runtime. label Oct 5, 2022
@mdempsky
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mdempsky commented Oct 5, 2022

Probably the same underlying issue, but affecting type assertions:

$ go run q.go
main.T does not implement I

$ cat q.go
package main

import "fmt"

func main() {
	var i any
	i = T(0)

	if _, ok := i.(I); !ok {
		fmt.Printf("%T does not implement I\n", i)
	}
}

type I interface{ m() interface{ I } }

type T int

func (T) m() interface{ I } { return nil }

var _ I = T(0) // static assert

@mdempsky mdempsky mentioned this issue Oct 8, 2022
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@mdempsky
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mdempsky commented Sep 6, 2023

Since Go 1.20, cmd/compile rejects both examples with "invalid recursive type: anonymous interface refers to itself".

@mdempsky mdempsky closed this as completed Sep 6, 2023
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compiler/runtime Issues related to the Go compiler and/or runtime. FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done.
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