Some expressions involve operands that are unevaluated. According to the C++ Standard, [expr], paragraph 8 [ISO/IEC 14882-2014]:
In some contexts, unevaluated operands appear. An unevaluated operand is not evaluated. An unevaluated operand is considered a full-expression. [Note: In an unevaluated operand, a non-static class member may be named (5.1) and naming of objects or functions does not, by itself, require that a definition be provided. — end note]
...
| SEI CERT C Coding Standard | EXP44-C. Do not rely on side effects in operands to sizeof, _Alignof, or _Generic |
Bibliography
| [ISO/IEC 14882-2014] | Clause 5, "Expressions" Subclause 20.2.5, "Function Template declval" |
...