cmd/gc: cannot use a named unsafe.Pointer as receiver type

[dvyukov]

parent: 17677:d7db8c804ffa tip, darwin/amd64

package main
import "unsafe"
type T unsafe.Pointer
func (t *T) foo() {}
func main() {}

/tmp/unsafe.go:4: invalid receiver type *T (*T / unsafe.Pointer)

[cznic]

Comment 1:

unsafe.Pointer[0] is a pointer type thus both T and *T are invalid receiver types. IMO
#workingAsIntended
> type Pointer *ArbitraryType
> type ArbitraryType int
  [0]: http://localhost:6060/pkg/unsafe/#Pointer

[ianlancetaylor]

Comment 3:

I agree that this seems to me to be working as intended.  Dmitriy, did you have a
specific use for a method on a named variant of unsafe.Pointer?

[dvyukov]

Comment 4:

Here is where I want to use it:
https://code.google.com/p/go/source/browse/src/pkg/sync/cond.go#110
unsafe.Pointer would have less casts than uintptr.
A similar use case would be:
type ConfigPtr unsafe.Pointer
func (p *ConfigPtr) GetCurrent() *Config {
    return (*Config)(atomic.LoadPointer(p))
}
func (p *ConfigPtr) UpdateCurrent(c *Config) {
    atomic.StorePointer(p, unsafe.Pointer(c))
}
var currentConfig ConfigPtr

[cznic]

Comment 5:

What about:
----
type ConfigPtr struct { unsafe.Pointer }
func (p *ConfigPtr) GetCurrent() *Config {
    return (*Config)(atomic.LoadPointer(p.Pointer))
}
func (p *ConfigPtr) UpdateCurrent(c *Config) {
    atomic.StorePointer(p.Pointer, unsafe.Pointer(c))
}
var currentConfig ConfigPtr
----
?

[dvyukov]

Comment 6:

Well, this is another possible workaround. I've used uintptr to work around this.
I am not saying that this is particularly important.
But it's nice that you can do 'type Foo int' instead of 'type Foo struct {foo int}'.

[dvyukov]

Comment 7:

Rob, Robert, please confirm whether it's allowed by spec or not. If the compiler does
correct thing according to spec, then we just close this, this is not a feature request.

[robpike]

Comment 8:

The spec does not exactly say that unsafe.Pointer is a pointer. It just says a pointer
can be converted to/from an unsafe.Pointer. However, an example defines it as
*ArbitraryType, as does the definition in the library docs. Thus, I claim that
unsafe.Pointer is a pointer and thus this is WorkingAsIntended.
Reopen if you disagree, but I think it's reasonably clear.

Status changed to WorkingAsIntended.

[dvyukov]

Comment 9:

OK, thanks. I am not disagree.