[C++] expression that is not constexpr

Reference: https://stackoverflow.com/a/65559457

ISO: https://eel.is/c++draft/expr.const#5

C++ abstract machine assumes that all expressions are constant expressions, and provides a list of conditions that if not satisfied will make the expression not a constant expression.

The rule is here:
An expression E is a core constant expression unless the evaluation of E, following the rules of the abstract machine ([intro.execution]), would evaluate one of the following: ...