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CWG3048 [stmt.expand] Zero "iterations" only possible for iterating and enumerating expansion statements #743
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I'm reading the above as "the specification of destructuring expansion statements needs a special case for N=0, because the syntactic rewrite is not well-formed for that case".
Reacted by S. B. Tam and A. JiangRight. Whether it needs it or not (and can stay as is) will defer to CWG.
- changed the title
[-][stmt.expand] Zero "iterations" only possible for iterating and enumerating expansion statements[/-][+]CWG3048 [stmt.expand] Zero "iterations" only possible for iterating and enumerating expansion statements[/+]on Aug 10, 2025 The section label is wrong, it should be [stmt.expand].
I think this resolution isn't quite right because it doesn't check all the semantic constraints that an empty structured binding declaration would have (if written using a pack in a templated context). For example, it doesn't check
constexprif present and it doesn't try to instantiatestd::tuple_size<E>. I think we're better off trying to find a way to say that we still treat it as a structured binding declaration even though it would be ill-formed to write it directly.In order to determine N, we need to instantiate
tuple_size, I think. [dcl.struct.bind] p7Yes. For the constexpr if present it could just be
constexpropt auto && range = expansion-initializer;
in the N == 0 case instead of(void) expansion-initializer;. Though, I think N should be introduced before it is used in the N == 0 case.I've fixed CWG3048. I haven't done anything about "N"; the general pattern is "rewrite ... where N is ..."
so N is already introduced too late.In order to determine N, we need to instantiate
tuple_size, I think. [dcl.struct.bind] p7Right, my bad. It looks like [dcl.struct.bind] checks all the constraints before telling us the structured binding size, so the proposed resolution looks good now.
Full name of submitter (unless configured in github; will be published with the issue): Jakub Jelinek
Reference (section label): stmt.expand
Link to reflector thread (if any):
Issue description: Right now both iterating and enumerating expansion statements can have zero "iterations", the former if begin is the same as end, the latter for {}. But for destructuring expansion statements, I believe it has to have at least one iteration, because a valid structured binding declaration needs to have at least one name. While with structured binding packs one can have zero, i.e. if ihttps://eel.is/c++draft/stmt.expand#5.3 was instead constexpropt auto&& [...u] = expansion-initializer; and for-range-declaration = u...[i]; then it would allow also zero "itereations". Though, structured binding packs can only appear in templates, so such a change isn't possible, it would need to be some special case.
Suggested resolution: