[Note 1:

An explicit specialization can differ from the template declaration with respect to the constexpr or consteval specifier.

— end note]

[Note 2:

Function parameters cannot be declared constexpr.

— end note]

[Example 1: constexpr void square(int &x); constexpr int bufsz = 1024; constexpr struct pixel { int x; int y; constexpr pixel(int); }; constexpr pixel::pixel(int a) : x(a), y(x) { square(x); } constexpr pixel small(2); constexpr void square(int &x) { x *= x; } constexpr pixel large(4); int next(constexpr int x) { return x + 1; } extern constexpr int memsz; — end example]

[Note 3:

A function declared with the consteval specifier is an immediate function ([expr.const]).

— end note]

Except for instantiated constexpr functions, non-templated constexpr functions shall be constexpr-suitable.

[Example 2: constexpr int square(int x) { return x * x; } constexpr long long_max() { return 2147483647; } constexpr int abs(int x) { if (x < 0) x = -x; return x; } constexpr int constant_non_42(int n) { if (n == 42) { static int value = n; return value; } return n; } constexpr int uninit() { struct { int a; } s; return s.a; } constexpr int prev(int x) { return --x; } constexpr int g(int x, int n) { int r = 1; while (--n > 0) r *= x; return r; } — end example]

An invocation of a constexpr function in a given context produces the same result as an invocation of an equivalent non-constexpr function in the same context in all respects except that

• an invocation of a constexpr function can appear in a constant expression ([expr.const]) and
• copy elision is not performed in a constant expression ([class.copy.elision]).

[Note 4:

Declaring a function constexpr can change whether an expression is a constant expression.

This can indirectly cause calls to std​::​is_constant_evaluated within an invocation of the function to produce a different value.

— end note]

[Note 5:

It is possible to write a constexpr function for which no invocation satisfies the requirements of a core constant expression.

— end note]

[Example 3: constexpr int bar(int x, int y) { return x + y + x*y; } int bar(int x, int y) { return x * 2 + 3 * y; } — end example]

[Example 4: struct pixel { int x, y; }; constexpr pixel ur = { 1294, 1024 }; constexpr pixel origin; namespace N { void f() { int x; constexpr int& ar = x; static constexpr int& sr = x; } } — end example]