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cmd/compile: build failed #50660

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cuiweixie opened this issue Jan 17, 2022 · 11 comments
Closed

cmd/compile: build failed #50660

cuiweixie opened this issue Jan 17, 2022 · 11 comments
Labels
FrozenDueToAge NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. release-blocker
Milestone
Go1.18

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@cuiweixie
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cuiweixie commented Jan 17, 2022 •
edited

What version of Go are you using (go version)?

$ go version
go version devel go1.18-897b3da2e0 Mon Jan 17 09:23:25 2022 +0000 darwin/arm64

Does this issue reproduce with the latest release?

no

What did you do?

example1 :


import (
	"fmt"
)


type I int64

func(i I) Do() int64 {
	return int64(i)
}

type Int interface {
	int32 | int64
}

type Doer[T Int] interface {
	Do() T
}

func Specialize[T Int](v Doer[T]) {
	switch v.(type) {
	case Doer[int32]:
		fmt.Println("int32!")
	case Doer[int64]:
		fmt.Println("int64!")
	}
}

func main() {
	var i Doer[int64]
	i = I(1)
	Specialize[int64](i)
}

example2:

package main

import (
	"fmt"
)


type I int64

func(i I) Do() int64 {
	return int64(i)
}

type Int interface {
	int32 | int64
}

type Doer[T Int] interface {
	Do() T
}

func Specialize[T Int](v Doer[T]) {
	switch v1 := v.(type) {
	case Doer[int32]:
		var v2 Doer[T]
		v2 = v1
		fmt.Println("int32!", v1)
	case Doer[int64]:
		fmt.Println("int64!")
	}
}

func main() {
	var i Doer[int64]
	i = I(1)
	Specialize[int64](i)
}

What did you expect to see?

example1:
go run success!
example2:
go run success!

What did you see instead?

example1:
go run success!
example2:

# command-line-arguments
./main.go:26:8: cannot use v1 (variable of type Doer[int32]) as type Doer[T] in assignment:
        Doer[int32] does not implement Doer[T] (wrong type for Do method)
                have Do() int32 at ./main.go:19:2
                want Do() T
@cuiweixie
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cuiweixie commented Jan 17, 2022 •
edited

when i try to fix #50658 , i found this will failed too.

@ianlancetaylor
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CC @griesemer @findleyr

Probably the same problem as #50658.

@ianlancetaylor ianlancetaylor added NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. release-blocker labels Jan 17, 2022
@ianlancetaylor ianlancetaylor added this to the Go1.18 milestone Jan 17, 2022
@erifan
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erifan commented Jan 18, 2022 

var v2 Doer[T]
v2 = v1

Should this be a "declared but not used" error ?

@ianlancetaylor
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@erifan Yes, looks like it. But that is a separate issue.

@erifan
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erifan commented Jan 18, 2022

@ianlancetaylor If we rule out this problem, then this issue and #50658 seem to be the same, then we can close this.

@ianlancetaylor
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The error message here (cannot use v1 (variable of type Doer[int32]) as type Doer[T] in assignment) is not the same as the error message in #50658 (impossible type assertion). I would not be surprised if they are the same underlying problem, but let's verify that before we close this issue. Thanks.

@cuiweixie
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run example1 success.
according to #50658 , example1 will failed.
#50658 failed in

go/src/cmd/vendor/golang.org/x/tools/go/analysis/passes/ifaceassert/ifaceassert.go

Line 98 in 31496cf

"impossible type assertion: no type can implement both %v and %v (conflicting types for %v method)",
that cause by go vet.
but this issue failed in cmd/compile.

@findleyr
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This is a different issue from #50658. In the line v2 = v1, v2 is a Doer[T], and v1 is a Doer[int32]. The type checker does not infer that T must be an int32 inside this case.

I don't think this is valid code with the current spec. Shouldn't it be:

	case Doer[int32]:
		var v2 Doer[int32]
		v2 = v1
		fmt.Println("int32!", v1)

?

@ianlancetaylor
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Ah, you're right. Sorry for misunderstanding. This is not a bug.

@erifan
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erifan commented Jan 20, 2022

The type checker does not infer that T must be an int32 inside this case.

Although this is not valid code according to the current spec, but it is clear that the value of T can be inferred under this case statement. So I think it might be more convenient if we make this code valid. Maybe not comparable, but the interface can do it, see https://go.dev/play/p/KqfF-6ElUnR

@findleyr
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it is clear that the value of T can be inferred under this case statement.

While it may be obvious in this case, I think if we were to write down a specification for this feature we'd find that there are many less obvious corner cases, and the resulting behavior may be surprising to users (in short, we'd have to do some sort of unification with types in type lists for type switch cases). My intuition is that it is better to just do the substitution explicitly where necessary, as in writing out var v2 Doer[int32] here.

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