tmp/tmp1hbglop1/{from.md → to.md}
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| 1 |
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#### Compound requirements <a id="expr.prim.req.compound">[[expr.prim.req.compound]]</a>
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``` bnf
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compound-requirement:
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'{' expression '}' noexceptₒₚₜ return-type-requirementₒₚₜ ';'
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```
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``` bnf
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return-type-requirement:
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'->' type-constraint
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```
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A *compound-requirement* asserts properties of the *expression* E.
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Substitution of template arguments (if any) and verification of semantic
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| 15 |
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properties proceed in the following order:
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- Substitution of template arguments (if any) into the *expression* is
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performed.
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- If the `noexcept` specifier is present, E shall not be a
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potentially-throwing expression [[except.spec]].
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- If the *return-type-requirement* is present, then:
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- Substitution of template arguments (if any) into the
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*return-type-requirement* is performed.
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- The immediately-declared constraint [[temp.param]] of the
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*type-constraint* for `decltype((E))` shall be satisfied.
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\[*Example 1*:
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Given concepts `C` and `D`,
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``` cpp
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requires {
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| 30 |
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{ E1 } -> C;
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{ E2 } -> D<A₁, ⋯, Aₙ>;
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};
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```
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is equivalent to
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``` cpp
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requires {
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E1; requires C<decltype((E1))>;
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E2; requires D<decltype((E2)), A₁, ⋯, Aₙ>;
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};
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```
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(including in the case where n is zero).
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— *end example*]
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| 45 |
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| 46 |
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[*Example 2*:
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| 47 |
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``` cpp
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template<typename T> concept C1 = requires(T x) {
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| 50 |
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{x++};
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};
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```
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| 53 |
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The *compound-requirement* in `C1` requires that `x++` is a valid
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expression. It is equivalent to the *simple-requirement* `x++;`.
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``` cpp
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template<typename T> concept C2 = requires(T x) {
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| 59 |
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{*x} -> std::same_as<typename T::inner>;
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};
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```
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The *compound-requirement* in `C2` requires that `*x` is a valid
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expression, that `typename T::inner` is a valid type, and that
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`std::same_as<decltype((*x)), typename T::inner>` is satisfied.
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| 66 |
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| 67 |
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``` cpp
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template<typename T> concept C3 =
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| 69 |
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requires(T x) {
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| 70 |
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{g(x)} noexcept;
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| 71 |
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};
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```
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The *compound-requirement* in `C3` requires that `g(x)` is a valid
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| 75 |
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expression and that `g(x)` is non-throwing.
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| 76 |
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| 77 |
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— *end example*]
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