Whether or not an expression not explicitly sanctioned by this
document is an extended constant expression, whether or not such
extended constant expressions can be used in the same contexts as this
document, and whether or not such extended constant expressions can
affect potentially detectable semantic changes in the program (C23
6.6).
The only extended constant expressions are those using other GNU
extensions to the C language. Such extended constant expressions
include integer constant expressions and arithmetic constant
expressions.