9 The constexpr and consteval specifiers

9.1 The constexpr and consteval specifiers [dcl.constexpr]

1. Except as modified by this clause, [dcl.constexpr] applies unchanged to a SCPP26 program.

2. The tokens constexpr and consteval have the meanings assigned by the C++ standard; this document introduces no additional spelling for compile-time-evaluable declarations.

3. A function or constructor declared constexpr is eligible for compile-time evaluation. If an invocation of such a function or constructor occurs in required constant evaluation (7.1), the invocation shall satisfy Clause 7. Otherwise, the invocation is an ordinary runtime invocation with the semantics the C++ standard gives it.

4. A function or constructor declared consteval is an immediate function. Every potentially-evaluated call to it shall produce a constant expression. If it does not, the program is ill-formed.

5. A destructor, allocation function, or deallocation function shall not be declared consteval.

6. A constexpr or consteval constructor may participate in required constant evaluation only for a class type satisfying 7.2(2).

[Note: this clause reuses the C++ spellings and meanings of constexpr and consteval. The SCPP26-specific restrictions are those of Clause 7’s supported constant-evaluation subset. — end note]

9.2 The if consteval statement [stmt.if]

1. An if statement of the form if consteval is a consteval if statement. An if statement of the form if !consteval is a negated consteval if statement.

2. Except as modified by this subclause, [stmt.if] applies unchanged to a consteval if statement or negated consteval if statement.

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

4. Otherwise, if the else part of a consteval if statement is present, the second substatement is executed.

5. If a negated consteval if statement is evaluated in a context that is manifestly constant-evaluated ([expr.const.defns]), the first substatement is not executed and, if the else part is present, the second substatement is executed.

6. Otherwise, the first substatement of a negated consteval if statement is executed.

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

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