13 Templates [temp]

13.5 Template constraints [temp.constr]

13.5.2 Constraints [temp.constr.constr]

13.5.2.4 Fold expanded constraint [temp.constr.fold]

1
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A fold expanded constraint is formed from a constraint C and a fold-operator which can either be && or ||.
A fold expanded constraint is a pack expansion ([temp.variadic]).
Let N be the number of elements in the pack expansion parameters ([temp.variadic]).
2
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A fold expanded constraint whose fold-operator is && is satisfied if it is a valid pack expansion and if or if for each i where in increasing order, C is satisfied when replacing each pack expansion parameter with the corresponding element.
No substitution takes place for any i greater than the smallest i for which the constraint is not satisfied.
3
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A fold expanded constraint whose fold-operator is || is satisfied if it is a valid pack expansion, , and if for i where in increasing order, there is a smallest i for which C is satisfied when replacing each pack expansion parameter with the corresponding element.
No substitution takes place for any i greater than the smallest i for which the constraint is satisfied.
4
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[Note 1: 
If the pack expansion expands packs of different size, then it is invalid and the fold expanded constraint is not satisfied.
— end note]
5
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Two fold expanded constraints are compatible for subsumption if their respective constraints both contain an equivalent unexpanded pack ([temp.over.link]).