[expr.prim.req.compound]

A compound-requirement asserts properties of the expression E.

Substitution of template arguments (if any) and verification of semantic properties proceed in the following order:

(1.1)
Substitution of template arguments (if any) into the expression is performed.
(1.2)
If the noexcept specifier is present, E shall not be a potentially-throwing expression ([except.spec]).
(1.3)
If the return-type-requirement is present, then:
- (1.3.1)
  Substitution of template arguments (if any) into the return-type-requirement is performed.
- (1.3.2)
  The immediately-declared constraint ([temp.param]) of the type-constraint for decltype((E)) shall be satisfied.
  [ Example
  :
  Given concepts C and D,
  requires { { E1 } -> C; { E2 } -> D<A $_{1}$ , ⋯, A $_{n}$ >; };
  is equivalent to
  requires { E1; requires C<decltype((E1))>; E2; requires D<decltype((E2)), A $_{1}$ , ⋯, A $_{n}$ >; };
  (including in the case where n is zero).
  — end example
  ]

template<typename T> concept C1 = requires(T x) {
  {x++};
};

The compound-requirement in C1 requires that x++ is a valid expression.

It is equivalent to the simple-requirement x++;.

template<typename T> concept C2 = requires(T x) {
  {*x} -> std::same_as<typename T::inner>;
};

The compound-requirement in C2 requires that *x is a valid expression, that typename T::inner is a valid type, and that std::same_as<decltype((*x)), typename T::inner> is satisfied.

template<typename T> concept C3 =
  requires(T x) {
    {g(x)} noexcept;
  };

The compound-requirement in C3 requires that g(x) is a valid expression and that g(x) is non-throwing.

— end example

]

7 Expressions [expr]

7.5 Primary expressions [expr.prim]

7.5.7 Requires expressions [expr.prim.req]

7.5.7.3 Compound requirements [expr.prim.req.compound]