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[expr.typeid] glvalue of incomplete type vs typeid #4826

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xmh0511 opened this issue Aug 19, 2021 · 4 comments · Fixed by #4827
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[expr.typeid] glvalue of incomplete type vs typeid #4826

xmh0511 opened this issue Aug 19, 2021 · 4 comments · Fixed by #4827
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@jensmaurer
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cwg Issue must be reviewed by CWG. not-editorial Issue is not deemed editorial; the editorial issue is kept open for tracking.

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@xmh0511
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xmh0511 commented Aug 19, 2021 

struct C;
int storage;
C* ptr = (C*)&storage;
typeid(*ptr);  //#1

All compilers report #1 is an error, with the diagnosis invalid use of incomplete type. However, there's no relevant rule in the standard that restricts the type of the glvalue operand in [expr.typeid]. Is it an omission?
@jwakely
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jwakely commented Aug 19, 2021 •
edited

Since it's unknown whether S is a polymorphic type or not, the behaviour here seems undefined. So giving an error is fine. Making it explicitly ill-formed would probably be better though.

@jensmaurer
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dynamic_cast consistently requires complete types for both source and target.

We have "If the type of the type-id is a class type or a reference to a class type, the class shall be completely-defined."

It seems plausible that this rule should also apply to the "expression" case.

@xmh0511
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xmh0511 commented Aug 19, 2021 •
edited

Maybe, we could say that

When typeid is applied to an expression, If the static type of the expression is a (possibly cv-qualified) class type, the class shall be completely-defined.

Which is before [expr.typeid] p2.

@jensmaurer jensmaurer mentioned this issue Aug 19, 2021
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@xmh0511
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xmh0511 commented Aug 19, 2021 •
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@jensmaurer For [expr.typeid] p3

If the expression is a prvalue, the temporary materialization conversion is applied. The expression is an unevaluated operand.

How about the following case?

typeid(static_cast<void>(0));

The operand is a prvalue of type void as per [expr.static.cast] p1 and [expr.static.cast] p6, which means the temporary materialization conversion is not possible to be applied to that operand. It is a bit contradictory.

@jensmaurer jensmaurer self-assigned this Aug 19, 2021
@jensmaurer jensmaurer added cwg Issue must be reviewed by CWG. not-editorial Issue is not deemed editorial; the editorial issue is kept open for tracking. labels Aug 19, 2021
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[expr.typeid] Require class type to be complete jensmaurer/draft
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@jwakely @xmh0511 @jensmaurer