Declared but not defined. Meta‐function use only. The tag can be used to recover meta‐information from an expression. Example: sizeof(f(expr).tag()) returns a different compile‐time value depending on the type of the result of calling the f function but does not actually call the f function or the tag method at run‐time. Note that f(expr)::VALUE or sizeof(f(expr)::Type) would be ill‐formed and that f(expr).value is not a compile‐time expression.