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reflect: document missing panic behavior for non-assignable values in reflect.Grow #70760

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gazerro opened this issue Dec 10, 2024 · 3 comments
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reflect: document missing panic behavior for non-assignable values in reflect.Grow #70760

gazerro opened this issue Dec 10, 2024 · 3 comments
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compiler/runtime Issues related to the Go compiler and/or runtime. Documentation Issues describing a change to documentation. NeedsFix The path to resolution is known, but the work has not been done.
Milestone
Go1.24

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@gazerro
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gazerro commented Dec 10, 2024

Currently, the documentation for reflect.Grow does not mention that the method panics if the value passed is not assignable:

It panics if v's Kind is not a Slice or if n is negative or too large to allocate the memory.

However, if v is not assignable (either because it was obtained using an unexported field or it is not addressable), Grow will also panic. For example:

package main

import (
	"reflect"
)

func main() {
	reflect.ValueOf([]int{1, 2, 3}).Grow(5)
}
@gopherbot gopherbot added compiler/runtime Issues related to the Go compiler and/or runtime. Documentation Issues describing a change to documentation. labels Dec 10, 2024
@randall77
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Same is true for the Set* methods, or Copy when the destination is an array.
We mention in some cases that they panic if CanSet is false, but fairly inconsistently.

@gazerro
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gazerro commented Dec 10, 2024

It’s not immediately obvious that Grow can panic for certain slices, such as in the following example, which correctly panics:

s := []int{1, 2, 3}
reflect.ValueOf(s).Grow(5)

I believe this behavior should be documented, along with the other cases mentioned by @randall77.

@gopherbot
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Change https://go.dev/cl/634799 mentions this issue: reflect: consistently document when value must be settable

@dmitshur dmitshur added this to the Go1.24 milestone Dec 11, 2024
@dmitshur dmitshur added the NeedsFix The path to resolution is known, but the work has not been done. label Dec 11, 2024
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compiler/runtime Issues related to the Go compiler and/or runtime. Documentation Issues describing a change to documentation. NeedsFix The path to resolution is known, but the work has not been done.
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