In `switch (count)`, `count` may vary, but each `case` value must be an integer constant expression (ICE). Values such as `case 4:` qualify; a variable such as `count` cannot supply a case value. Case values must also be unique within the switch. section 3.3
An ICE provides an integer value determinable during translation. That is not enough by itself: a function call cannot supply the value, and an ordinary object cannot participate in its evaluation, even if it is `const`-qualified. Integer literals and enumeration constants can be used; an object declared with `constexpr` can also participate. section 5.6.2
signed const o42 = 42;
constexpr signed c42 = 42;
enum {
b42 = 42, // Ok: 42 is a literal.
c52 = o42 + 10, // Error: o42 is an object.
b52 = b42 + 10, // Ok: b42 is not an object.
d52 = c42 + 10, // Ok: c42 is a named constant.
};
Compare `b52`, formed from an enumeration constant, with the invalid `c52`, formed from a `const` object. `d52` is valid because `c42` is declared with `constexpr`. section 5.6.2
Enumeration constants can name values used in further enumeration initializers. An enumeration count can also appear as an array bound, avoiding a separately tracked numeric size. TAKEAWAY: A value being known to a reader—or stored in a `const` object—does not alone make its expression an ICE. section 5.6.2
