From cppreference.com
template<classT>conceptdefault_initializable=std::constructible_from<T>&&requires{T{};}&&/* T t; is well-formed, see below */;(since C++20)The default_initializable concept checks whether variables of type T can be
(i.e., whether T() is well-formed);
from an empty initializer list (i.e., whether T{} is well-formed); and
(i.e., whether Tt; is well-formed).
Access checking is performed as if in a context unrelated to T. Only the validity of the immediate context of the variable initialization is considered.
Possible implementation
template<classT>conceptdefault_initializable=std::constructible_from<T>&&requires{T{};::newT;};References
C++23 standard (ISO/IEC 14882:2024):
18.4.12 Concept default_initializable [concept.default.init]
C++20 standard (ISO/IEC 14882:2020):
18.4.12 Concept default_initializable [concept.default.init]
See also