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runtime: wrong type assertion result when using generic types #51700

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disksing opened this issue Mar 16, 2022 · 8 comments
Closed

runtime: wrong type assertion result when using generic types #51700

disksing opened this issue Mar 16, 2022 · 8 comments
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FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done. release-blocker
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@disksing
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What version of Go are you using (go version)?

$ go version
go version go1.18 darwin/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

doesn't matter. It can reproduce on the go playground.

go env Output
$ go env

What did you do?

https://go.dev/play/p/A9VuqCpJ7r5

What did you expect to see?

As S does not have method name M1 or M2, it should not panic.

What did you see instead?

Panic and output iface1. I also noticed that if I rename IFace2 to AFace2, the result is panic and output iface2.

@griesemer
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Simplified version (playground):

package main

func f[B any](b B) {
	if b1, ok := any(b).(interface{ m1() }); ok {
		panic(1)
		_ = b1.(B) // <<< removing this type assertion makes this code work
	}
	if b2, ok := any(b).(interface{ m2() }); ok {
		panic(2)
		_ = b2.(B) // <<< removing this type assertion makes this code work
	}
}

type S struct{}

func (S) m3() {}

func main() {
	f(S{})
}

Replacing the type parameter B with S produces the expected result (no panic).
cc: @mdempsky (for trying with unified IR), @randall77

@mdempsky
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Test cases run correctly (i.e., exit quietly with success) with GOEXPERIMENT=unified.

@heschi heschi added the NeedsFix The path to resolution is known, but the work has not been done. label Mar 16, 2022
@heschi heschi added this to the Go1.19 milestone Mar 16, 2022
@heschi
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heschi commented Mar 16, 2022

Tentatively marking release blocker.

@mdempsky
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S and m3 aren't even needed. Just f(0) repros the issue too.

@mdempsky
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It seems to happen for any named type. E.g., instantiating B as []int doesn't cause problems, but instantiating it as SliceInt does.

@ianlancetaylor
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@gopherbot Please open backport to 1.18.

This appears to be incorrect code generation, so we should backport the fix when we have one.

@gopherbot
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Backport issue(s) opened: #51738 (for 1.18).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.

@dmitshur dmitshur changed the title go/runtime: wrong type assertion result when using generic types runtime: wrong type assertion result when using generic types Apr 6, 2022
@gopherbot
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Change https://go.dev/cl/399058 mentions this issue: cmd/compile: always write fun[0] in incompelte itab

@golang golang locked and limited conversation to collaborators Apr 12, 2023
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