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cmd/compile: can't assign pointer from generic type to generic interface satisfied by pointer receiver #47747

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4ad opened this issue Aug 17, 2021 · 6 comments
Closed

cmd/compile: can't assign pointer from generic type to generic interface satisfied by pointer receiver #47747

4ad opened this issue Aug 17, 2021 · 6 comments
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FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done.
Milestone
Go1.18

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@4ad
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4ad commented Aug 17, 2021 •
edited

On tip (a304273) with -gcflags -G=3.

Repro:

package p

type Fooer[t any] interface {
	Foo(Barer[t])
}
type Barer[t any] interface {
	Bar(t)
}

type Foo[t any] t
type Bar[t any] t

func (l Foo[t]) Foo(v Barer[t]) { v.Bar(t(l)) }
func (b *Bar[t]) Bar(l t)       { *b = Bar[t](l) }

func f[t any](f Fooer[t]) t {
	var b Bar[t]
	f.Foo(&b)
	return t(b)
}

Compiler aborts with:

go build -gcflags -G=3 g.go
# command-line-arguments
./g.go:18:8: incompatible type: cannot use &b (value of type *Bar[t]) as Barer[t] value

Works with go2go: https://go2goplay.golang.org/p/HKqbj1Y6Ahh
@4ad 4ad added this to the Go1.18 milestone Aug 17, 2021
@4ad
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4ad commented Aug 17, 2021 •
edited

Simpler repro:

package p

type Fooer[t any] interface {
	Foo()
}

type Foo[t any] t

func (f *Foo[t]) Foo() {}

func f[t any](v t) {
	var f = Foo[t](v)
	_ = Fooer[t](&f)
}


go build -gcflags -G=3 g.go
# command-line-arguments
./g.go:13:15: cannot convert &f (value of type *Foo[t]) to Fooer[t]

Works with go2go: https://go2goplay.golang.org/p/TSQHwwv_p3o

@4ad 4ad changed the title cmd/compile: compiler rejects assignment of value of generic type to satisfied interface cmd/compile: can't assign pointer from generic type to generic interface satisfied by pointer receiver Aug 17, 2021
@mknyszek
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CC @ianlancetaylor @griesemer because I'm not actually sure what the rules are around this case.

@mknyszek mknyszek added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Aug 17, 2021
@griesemer
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Simpler reproducer: https://go2goplay.golang.org/p/NLPVJOJuWIY (works with go2go but not with current compiler).

@griesemer griesemer self-assigned this Aug 17, 2021
@griesemer
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Direct reproducer:

package p

type T[P any] P

func (T[_]) m() {}

func _[P any](x *T[P]) {
        x.m()
}

@griesemer griesemer added NeedsFix The path to resolution is known, but the work has not been done. and removed NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. labels Aug 17, 2021
@gopherbot
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Change https://golang.org/cl/342990 mentions this issue: cmd/compile/internal/types2: fix lookup for type-parameter based type

@gopherbot
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Change https://golang.org/cl/342991 mentions this issue: go/types: fix method lookup for type-parameter based types

gopherbot pushed a commit that referenced this issue Aug 17, 2021
@griesemer
cmd/compile/internal/types2: fix method lookup for type-parameter bas…
9d9e329 
…ed types

Pointers to type parameters don't have methods, but pointers to
defined types whose underlying types are type parameters may have
methods. Fix the respective test.

For #47747.

Change-Id: I1de47be094ed9297f0e7782538011657c37c5adc
Reviewed-on: https://go-review.googlesource.com/c/go/+/342990
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
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@mknyszek @4ad @griesemer @gopherbot