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[basic.scope.pdecl]/1 Example is outdated #3578

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languagelawyer opened this issue Dec 25, 2019 · 1 comment · Fixed by #3580
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[basic.scope.pdecl]/1 Example is outdated #3578

languagelawyer opened this issue Dec 25, 2019 · 1 comment · Fixed by #3580
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@jensmaurer

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@languagelawyer
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languagelawyer commented Dec 25, 2019 •
edited

From https://stackoverflow.com/questions/59442521/use-of-variable-in-own-initializer
After the resolution of CWG2256¹ changed the rules of an object lifetime start, unsigned char x = x; no longer initializes x with its own indeterminate value as the Example in [basic.scope.pdecl]/1 says, but triggers undefined behavior, because the initializer accesses the object whose lifetime has not started yet (because variable initialization hasn't completed).

  1. Which, if ignore references, kinda mixes variables with objects…
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jensmaurer commented Dec 25, 2019 •
edited

So, the definition of "vacuous initialization" has changed, and that means "unsigned char x = x" no longer has vacuous initialization, thus the glvalue "x" is used to access an object whose lifetime has not yet begun, which is undefined behavior. If that's unintended (is it?), that doesn't seem to be editorial at all to me.

If the change in semantics here is intended, fixing the example is indeed editorial.

Asked CWG: http://lists.isocpp.org/core/2019/12/8017.php

@jensmaurer jensmaurer added the cwg Issue must be reviewed by CWG. label Dec 25, 2019
@jensmaurer jensmaurer mentioned this issue Dec 26, 2019
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@jensmaurer jensmaurer removed the cwg Issue must be reviewed by CWG. label Dec 26, 2019
@jensmaurer jensmaurer self-assigned this Dec 26, 2019
@shafik shafik mentioned this issue Apr 13, 2023
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[basic.scope.pdecl] Fix example of self-referential initialization. jensmaurer/draft
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