[stmt.if]

If the if statement is of the form if constexpr, the value of the condition is contextually converted to bool and the converted expression shall be a constant expression ([expr.const]); this form is called a constexpr if statement.

If the value of the converted condition is false, the first substatement is a discarded statement, otherwise the second substatement, if present, is a discarded statement.

During the instantiation of an enclosing templated entity ([temp.pre]), if the condition is not value-dependent after its instantiation, the discarded substatement (if any) is not instantiated.

Each substatement of a constexpr if statement is a control-flow-limited statement ([stmt.label]).

[Example 1: if constexpr (sizeof(int[2]) {} / OK, narrowing allowed — end example]

[Note 1:

Odr-uses ([basic.def.odr]) in a discarded statement do not require an entity to be defined.

— end note]

[Example 2: template<typename T, typename .. Rest> void g(T&& p, Rest&& ..rs) { / ... handle p if constexpr (sizeof.(rs) > 0) g(rs..); / never instantiated with an empty argument list } extern int x; / no definition of x required int f() { if constexpr (true) return 0; else if (x) return x; else return -x; } — end example]

An if statement of the form

if constexpr

{o p t (init-statement condition) statement}_{o p t (init-statement condition) statement}

is equivalent to

{
init-statement
if constexpr

{o p t (condition) statement}}_{o p t (condition) statement}}

and an if statement of the form

if constexpr

{o p t (init-statement condition) statement else statement}_{o p t (init-statement condition) statement else statement}

is equivalent to

{
init-statement
if constexpr

{o p t (condition) statement else statement}}_{o p t (condition) statement else statement}}

except that the init-statement is in the same scope as the condition.

An if statement of the form if consteval is called a consteval if statement.

The statement, if any, in a consteval if statement shall be a compound-statement.

[Example 3: constexpr void f(bool b) { if (true) if consteval { } else ; / error: not a compound-statement; else not associated with outer if } — end example]

If a consteval if statement is evaluated in a context that is manifestly constant-evaluated ([expr.const]), the first substatement is executed.

[Note 2:

The first substatement is an immediate function context.

— end note]

Otherwise, if the else part of the selection statement is present, then the second substatement is executed.

Each substatement of a consteval if statement is a control-flow-limited statement ([stmt.label]).

An if statement of the form

if ! consteval compound-statement

is not itself a consteval if statement, but is equivalent to the consteval if statement

if consteval { } else compound-statement

An if statement of the form

if ! consteval compound-statement

{1 else statement 2_{2}}_{1 else statement 2_{2}}

is not itself a consteval if statement, but is equivalent to the consteval if statement

if consteval statement

{2 else compound-statement 1_{1}}_{2 else compound-statement 1_{1}}

68)68)

In other words, the else is associated with the nearest un-elsed if.