[exec.read.env]

read_env is a customization point object.

For some query object q, the expression read_env(q) is expression-equivalent to make-sender(read_env, q).

The exposition-only class template impls-for ([exec.snd.expos]) is specialized for read_env as follows:

namespace std::execution { template<> struct impls-for<decayed-typeof<read_env>> : default-impls { static constexpr auto start = [](auto query, auto& rcvr) noexcept -> void { TRY-SET-VALUE(rcvr, query(get_env(rcvr)); }; }; template<class Sndr, class Env> static consteval void check-types(); }

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template<class Sndr, class Env>
  static consteval void check-types();

Throws: An exception of type unspecified-exception ([exec.snd.general]) if the expression Q()(env) is ill-formed or has type void, where env is an lvalue subexpression whose type is Env.