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[stmt.select]

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  ## Selection statements <a id="stmt.select">[[stmt.select]]</a>
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  Selection statements choose one of several flows of control.
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5
  ``` bnf
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  selection-statement:
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  if constexprₒₚₜ '(' init-statementₒₚₜ condition ')' statement
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  if constexprₒₚₜ '(' init-statementₒₚₜ condition ')' statement else statement
 
 
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  switch '(' init-statementₒₚₜ condition ')' statement
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  ```
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12
  See  [[dcl.meaning]] for the optional *attribute-specifier-seq* in a
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  condition.
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  [*Note 1*: An *init-statement* ends with a semicolon. — *end note*]
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- The substatement in a *selection-statement* (each substatement, in the
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- `else` form of the `if` statement) implicitly defines a block scope
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- [[basic.scope]]. If the substatement in a *selection-statement* is a
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- single statement and not a *compound-statement*, it is as if it was
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- rewritten to be a *compound-statement* containing the original
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- substatement.
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-
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- [*Example 1*:
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-
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- ``` cpp
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- if (x)
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- int i;
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- ```
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-
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- can be equivalently rewritten as
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-
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- ``` cpp
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- if (x) {
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- int i;
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- }
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- ```
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-
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- Thus after the `if` statement, `i` is no longer in scope.
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-
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- — *end example*]
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43
  ### The `if` statement <a id="stmt.if">[[stmt.if]]</a>
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- If the condition [[stmt.select]] yields `true` the first substatement is
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  executed. If the `else` part of the selection statement is present and
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  the condition yields `false`, the second substatement is executed. If
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  the first substatement is reached via a label, the condition is not
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  evaluated and the second substatement is not executed. In the second
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  form of `if` statement (the one including `else`), if the first
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  substatement is also an `if` statement then that inner `if` statement
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  shall contain an `else` part.[^1]
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  If the `if` statement is of the form `if constexpr`, the value of the
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- condition shall be a contextually converted constant expression of type
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- `bool` [[expr.const]]; this form is called a *constexpr if* statement.
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- If the value of the converted condition is `false`, the first
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- substatement is a *discarded statement*, otherwise the second
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- substatement, if present, is a discarded statement. During the
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- instantiation of an enclosing templated entity [[temp.pre]], if the
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- condition is not value-dependent after its instantiation, the discarded
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- substatement (if any) is not instantiated.
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-
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- [*Note 1*: Odr-uses [[basic.def.odr]] in a discarded statement do not
 
 
 
 
 
 
 
 
 
 
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  require an entity to be defined. — *end note*]
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- A `case` or `default` label appearing within such an `if` statement
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- shall be associated with a `switch` statement [[stmt.switch]] within the
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- same `if` statement. A label [[stmt.label]] declared in a substatement
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- of a constexpr if statement shall only be referred to by a statement
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- [[stmt.goto]] in the same substatement.
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-
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- [*Example 1*:
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75
  ``` cpp
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  template<typename T, typename ... Rest> void g(T&& p, Rest&& ...rs) {
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  // ... handle p
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@@ -122,25 +108,80 @@ is equivalent to
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  init-statement
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  if constexprₒₚₜ '(' condition ')' statement else statement
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  '}'
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  ```
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127
- except that names declared in the *init-statement* are in the same
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- declarative region as those declared in the *condition*.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
129
 
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  ### The `switch` statement <a id="stmt.switch">[[stmt.switch]]</a>
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  The `switch` statement causes control to be transferred to one of
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  several statements depending on the value of a condition.
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- The condition shall be of integral type, enumeration type, or class
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- type. If of class type, the condition is contextually implicitly
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- converted [[conv]] to an integral or enumeration type. If the (possibly
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- converted) type is subject to integral promotions [[conv.prom]], the
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- condition is converted to the promoted type. Any statement within the
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- `switch` statement can be labeled with one or more case labels as
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- follows:
 
 
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  ``` bnf
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  case constant-expression ':'
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  ```
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@@ -191,8 +232,8 @@ is equivalent to
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  init-statement
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  switch '(' condition ')' statement
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  '}'
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  ```
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- except that names declared in the *init-statement* are in the same
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- declarative region as those declared in the *condition*.
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1
  ## Selection statements <a id="stmt.select">[[stmt.select]]</a>
2
 
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+ ### General <a id="stmt.select.general">[[stmt.select.general]]</a>
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+
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  Selection statements choose one of several flows of control.
6
 
7
  ``` bnf
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  selection-statement:
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  if constexprₒₚₜ '(' init-statementₒₚₜ condition ')' statement
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  if constexprₒₚₜ '(' init-statementₒₚₜ condition ')' statement else statement
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+ if '!'ₒₚₜ consteval compound-statement
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+ if '!'ₒₚₜ consteval compound-statement else statement
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  switch '(' init-statementₒₚₜ condition ')' statement
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  ```
15
 
16
  See  [[dcl.meaning]] for the optional *attribute-specifier-seq* in a
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  condition.
18
 
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  [*Note 1*: An *init-statement* ends with a semicolon. — *end note*]
20
 
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+ [*Note 2*: Each *selection-statement* and each substatement of a
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+ *selection-statement* has a block scope
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+ [[basic.scope.block]]. — *end note*]
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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  ### The `if` statement <a id="stmt.if">[[stmt.if]]</a>
26
 
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+ If the condition [[stmt.pre]] yields `true` the first substatement is
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  executed. If the `else` part of the selection statement is present and
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  the condition yields `false`, the second substatement is executed. If
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  the first substatement is reached via a label, the condition is not
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  evaluated and the second substatement is not executed. In the second
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  form of `if` statement (the one including `else`), if the first
33
  substatement is also an `if` statement then that inner `if` statement
34
  shall contain an `else` part.[^1]
35
 
36
  If the `if` statement is of the form `if constexpr`, the value of the
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+ condition is contextually converted to `bool` and the converted
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+ expression shall be a constant expression [[expr.const]]; this form is
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+ called a *constexpr if* statement. If the value of the converted
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+ condition is `false`, the first substatement is a *discarded statement*,
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+ otherwise the second substatement, if present, is a discarded statement.
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+ During the instantiation of an enclosing templated entity [[temp.pre]],
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+ if the condition is not value-dependent after its instantiation, the
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+ discarded substatement (if any) is not instantiated. Each substatement
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+ of a constexpr if statement is a control-flow-limited statement
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+ [[stmt.label]].
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+
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+ [*Example 1*:
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+
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+ ``` cpp
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+ if constexpr (sizeof(int[2])) {} // OK, narrowing allowed
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+ ```
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+
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+ — *end example*]
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+
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+ [*Note 1*: Odr-uses [[term.odr.use]] in a discarded statement do not
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  require an entity to be defined. — *end note*]
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59
+ [*Example 2*:
 
 
 
 
 
 
60
 
61
  ``` cpp
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  template<typename T, typename ... Rest> void g(T&& p, Rest&& ...rs) {
63
  // ... handle p
64
 
 
108
  init-statement
109
  if constexprₒₚₜ '(' condition ')' statement else statement
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  '}'
111
  ```
112
 
113
+ except that the *init-statement* is in the same scope as the
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+ *condition*.
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+
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+ An `if` statement of the form `if consteval` is called a
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+ *consteval if statement*. The *statement*, if any, in a consteval if
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+ statement shall be a *compound-statement*.
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+
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+ [*Example 3*:
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+
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+ ``` cpp
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+ constexpr void f(bool b) {
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+ if (true)
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+ if consteval { }
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+ else ; // error: not a compound-statement; else not associated with outer if
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+ }
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+ ```
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+
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+ — *end example*]
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+
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+ If a consteval if statement is evaluated in a context that is manifestly
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+ constant-evaluated [[expr.const]], the first substatement is executed.
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+
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+ [*Note 2*: The first substatement is an immediate function
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+ context. — *end note*]
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+
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+ Otherwise, if the `else` part of the selection statement is present,
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+ then the second substatement is executed. Each substatement of a
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+ consteval if statement is a control-flow-limited statement
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+ [[stmt.label]].
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+
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+ An `if` statement of the form
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+
145
+ ``` bnf
146
+ if '!' consteval compound-statement
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+ ```
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+
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+ is not itself a consteval if statement, but is equivalent to the
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+ consteval if statement
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+
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+ ``` bnf
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+ if consteval '{' '}' else compound-statement
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+ ```
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+
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+ An `if` statement of the form
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+
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+ ``` bnf
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+ if '!' consteval compound-statement₁ else statement₂
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+ ```
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+
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+ is not itself a consteval if statement, but is equivalent to the
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+ consteval if statement
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+
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+ ``` bnf
166
+ if consteval statement₂ else compound-statement₁
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+ ```
168
 
169
  ### The `switch` statement <a id="stmt.switch">[[stmt.switch]]</a>
170
 
171
  The `switch` statement causes control to be transferred to one of
172
  several statements depending on the value of a condition.
173
 
174
+ The value of a *condition* that is an initialized declaration is the
175
+ value of the declared variable, or the value of the *expression*
176
+ otherwise. The value of the condition shall be of integral type,
177
+ enumeration type, or class type. If of class type, the condition is
178
+ contextually implicitly converted [[conv]] to an integral or enumeration
179
+ type. If the (possibly converted) type is subject to integral promotions
180
+ [[conv.prom]], the condition is converted to the promoted type. Any
181
+ statement within the `switch` statement can be labeled with one or more
182
+ case labels as follows:
183
 
184
  ``` bnf
185
  case constant-expression ':'
186
  ```
187
 
 
232
  init-statement
233
  switch '(' condition ')' statement
234
  '}'
235
  ```
236
 
237
+ except that the *init-statement* is in the same scope as the
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+ *condition*.
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