tmp/tmp6um98ouk/{from.md → to.md}
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| 1 |
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#### Nested requirements <a id="expr.prim.req.nested">[[expr.prim.req.nested]]</a>
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``` bnf
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nested-requirement:
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requires constraint-expression ';'
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```
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A *nested-requirement* can be used to specify additional constraints in
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terms of local parameters. The *constraint-expression* shall be
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satisfied [[temp.constr.decl]] by the substituted template arguments, if
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any. Substitution of template arguments into a *nested-requirement* does
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not result in substitution into the *constraint-expression* other than
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as specified in [[temp.constr.constr]].
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[*Example 1*:
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``` cpp
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template<typename U> concept C = sizeof(U) == 1;
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template<typename T> concept D = requires (T t) {
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requires C<decltype (+t)>;
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};
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```
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`D<T>` is satisfied if `sizeof(decltype (+t)) == 1`
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[[temp.constr.atomic]].
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— *end example*]
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A local parameter shall only appear as an unevaluated operand
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[[expr.prop]] within the *constraint-expression*.
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[*Example 2*:
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``` cpp
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template<typename T> concept C = requires (T a) {
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requires sizeof(a) == 4; // OK
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requires a == 0; // error: evaluation of a constraint variable
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};
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```
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— *end example*]
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