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cmd/compile: identical generic types compare unequal #49245

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rogpeppe opened this issue Oct 31, 2021 · 3 comments
Closed

cmd/compile: identical generic types compare unequal #49245

rogpeppe opened this issue Oct 31, 2021 · 3 comments
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generics

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@rogpeppe
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@rogpeppe rogpeppe commented Oct 31, 2021

commit 8e3d5f0

This bug was found by @FiloSottile in https://go-review.googlesource.com/c/go/+/360015.

Here's a smaller testscript reproducer:

go run ./pkg1

-- go.mod --
module m

go 1.18
-- pkg1/main.go --
package main

import (
	"fmt"
	"os"
	"reflect"

	"m/pkg2"
	"m/pkg3"
)

func main() {
	i1 := pkg3.GetI()
	i2 := pkg2.Inst
	instEqual := i1 == i2
	if !instEqual {
		v1, v2 := reflect.ValueOf(i1), reflect.ValueOf(i2)
		ptrEqual := v1.Pointer() == v2.Pointer()
		typeEqual := v1.Type() == v2.Type()
		if !ptrEqual {
			fmt.Printf("pointer values are unequal\n")
		}
		if !typeEqual {
			fmt.Printf("types are not equal\n\t%v\n\t%v\n", v1.Type(), v2.Type())
		}
		os.Exit(1)
	}
}

-- pkg2/x.go --
package pkg2

import "m/pkg3"

var Inst = pkg3.GetI()

-- pkg3/x.go --
package pkg3

type I interface{}

type impl[P any] struct{
	_ byte
}

var instance = &impl[int]{}

func GetI() I {
	return instance
}

Running this prints the following:

> go run ./pkg1
[stdout]
types are not equal
	*pkg3.impl[int]
	*pkg3.impl[int]

[stderr]
exit status 1

The values are not equal when they should be. The pointer values are identical but the types are not.

@rogpeppe rogpeppe added the generics label Oct 31, 2021
@seankhliao
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@seankhliao seankhliao commented Oct 31, 2021

is this the same as #49241 ?

@mvdan
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@mvdan mvdan commented Oct 31, 2021

I think it's the same issue, but @rogpeppe wrote a minimal reproducer, so perhaps this is still useful. cc @randall77 @griesemer

@FiloSottile
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@FiloSottile FiloSottile commented Oct 31, 2021

Yep, merging into #49241 which has more discussion, but thank you @rogpeppe for preparing a much better issue and reproducer.

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