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go/types, types2: append within a generic function does not compile #50281

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stellirin opened this issue Dec 21, 2021 · 6 comments
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go/types, types2: append within a generic function does not compile #50281

stellirin opened this issue Dec 21, 2021 · 6 comments
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FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done.
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@stellirin
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What version of Go are you using (go version)?

$ go version
go version go1.18beta1 linux/amd64

Does this issue reproduce with the latest release?

latest beta release

What did you do?

A generic function with a union constraint of []byte | string cannot append to a []byte.

func AppendBytes(buf []byte, s []byte) []byte {
	return append(buf, s[1:6]...)
}

func AppendString(buf []byte, s string) []byte {
	return append(buf, s[1:6]...)
}

type byteseq interface {
	string | []byte
}

// This should allow to eliminate the two functions above.
func AppendByteString[source byteseq](buf []byte, s source) []byte {
	return append(buf, s[1:6]...)
}

https://gotipplay.golang.org/p/WGG7g3HTfZ-

I expect this is a duplicate of a known issue but so far I was not able to find anytinhg. Similar functions are explicitly mentioned in the proposal: https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#composite-types-in-constraints

What did you expect to see?

The program compiles.

What did you see instead?

./prog.go:18:22: cannot use s[1:6] (value of type source constrained by byteseq) as type []byte in argument to append:
	cannot assign string (in source) to []byte
@ianlancetaylor ianlancetaylor added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Dec 21, 2021
@ianlancetaylor ianlancetaylor added this to the Go1.18 milestone Dec 21, 2021
@ianlancetaylor
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CC @griesemer @findleyr

@zhangfannie
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zhangfannie commented Dec 21, 2021

I modified the above program, see https://gotipplay.golang.org/p/j31fyLkpZcL, it works now.

func AppendByteString[source byteseq](buf []byte, s source) []byte{
	return append(buf, []byte(s[1:6])...)
}

Because I am not familiar with type parameters, I do not know if this modification changes its function. Thank you.

@zhangfannie
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Please ingore the above reply, the above mofication changes its function. 🙂

@aarzilli
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Isn't this already disallowed by the current spec because byteseq doesn't have a specific type?

@findleyr findleyr changed the title append within a generic function does not compile go/types, types2: append within a generic function does not compile Dec 21, 2021
@findleyr findleyr self-assigned this Dec 21, 2021
@griesemer
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The simple case is

package p

func _[S string|[]byte](s S) {
        var buf []byte
	_ = append(buf, s...)
}

which is currently disallowed because S has no structural type. But the spec has not been updated with respect to append, and we should have a special case here as we have for copy.

@griesemer griesemer assigned griesemer and unassigned findleyr Jan 7, 2022
@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 Jan 7, 2022
@gopherbot
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Change https://golang.org/cl/376214 mentions this issue: cmd/compile: accept string|[]byte-constrained 2nd argument in append

jproberts pushed a commit to jproberts/go that referenced this issue Jun 21, 2022
Similarly to what we do for the built-in function `copy`,
where we allow a string as 2nd argument to append, also
permit a type parameter constrained by string|[]byte.

While at it, change date in the manual.go2 test files so
that we don't need to constantly correct it when copying
a test case from that file into a proper test file.

Fixes golang#50281.

Change-Id: I23fed66736aa07bb3c481fe97313e828425ac448
Reviewed-on: https://go-review.googlesource.com/c/go/+/376214
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
@golang golang locked and limited conversation to collaborators Jun 22, 2023
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