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[class.copy.ctor] p10 What is the selected one by overload resolution for a variant member in the context of implicitly copy/move constructor for a union? #5194

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xmh0511 opened this issue Jan 10, 2022 · 2 comments
Open

[class.copy.ctor] p10 What is the selected one by overload resolution for a variant member in the context of implicitly copy/move constructor for a union? #5194

xmh0511 opened this issue Jan 10, 2022 · 2 comments

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@xmh0511
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xmh0511 commented Jan 10, 2022

[class.copy.ctor] p10 states that

An implicitly-declared copy/move constructor is an inline public member of its class. A defaulted copy/​move constructor for a class X is defined as deleted ([dcl.fct.def.delete]) if X has:

  • [...]
  • a variant member whose corresponding constructor as selected by overload resolution is non-trivial,

Since in [class.copy.ctor] p15, we have said that

The implicitly-defined copy/move constructor for a union X copies the object representation ([basic.types]) of X.

According to [basic.types.general] p4

The object representation of an object of type T is the sequence of N unsigned char objects taken up by the object of type T, where N equals sizeof(T).

Hence, copying the object representation of X means that copies the sequence of N unsigned char objects taken up by the object of type X, where N equals sizeof(X). So, there is no overload resolution here since we didn't invoke any constructor(function). This is a contradictory point. Should we say that?

a variant member whose corresponding constructor, as selected by overload resolution as if the variant member is copy-initialized from the corresponding source

As if the overload resolution can occur in this context to determine the constructor for the variant member.
@frederick-vs-ja
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I think the main problem is that the overload resolution is not clearly specified, because the sources of initialization are specified in [class.copy.ctor] p14, which only covers non-union classes.

IMO we can clarify the overload resolution by adding this to [class.copy.ctor] (10.2):

, where the overload resolution is performed as if the class of the member is not a union,

It might be better to separate the "sources of initialization" portion from [class.copy.ctor] p14.

Why do you choose "copy-initialized"? Only "direct-initialized" is used in [class.copy.ctor] p14.

@xmh0511
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xmh0511 commented Jan 11, 2022

@frederick-vs-ja Alright, I misused "copy-initialized". It should be "direct-initialized", in the manner as [class.copy.ctor] p14 did. I think this revision would be more clear:

a variant member whose corresponding constructor, as selected by overload resolution where the overload resolution is performed as if the class of which the variant is a member is a non-union class.

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@xmh0511 @frederick-vs-ja