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[expr.context]
7 Expressions [expr]
7.2 Properties of expressions [expr.prop]
7.2.3 Context dependence [expr.context]
In some contexts, unevaluated operands appear ([expr.prim.req.simple], [expr.prim.req.compound], [expr.typeid], [expr.sizeof], [expr.unary.noexcept], [expr.reflect], [dcl.type.decltype], [temp.pre], [temp.concept]).
An unevaluated operand is not evaluated.
[Note 1:
In an unevaluated operand, a non-static class member can be named ([expr.prim.id]) and naming of objects or functions does not, by itself, require that a definition be provided ([basic.def.odr]).
An unevaluated operand is considered a full-expression.
â end note]
In some contexts, an expression only appears for its side effects.
Such an expression is called a discarded-value expression.
The array-to-pointer and function-to-pointer standard conversions are not applied.
The lvalue-to-rvalue conversion is applied if and only if the expression is a glvalue of volatile-qualified type and it is one of the following:
( expression ), whereexpression is one of these expressions,
id-expression ([expr.prim.id]),
splice-expression ([expr.prim.splice]),
conditional expression where both the second and the third operands are one of these expressions, or
comma expression where the right operand is one of these expressions.
[Note 2:
Using an overloaded operator causes a function call; the above covers only operators with built-in meaning.
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The temporary materialization conversion ([conv.rval]) is applied if the (possibly converted) expression is a prvalue of object type.
[Note 3:
If the original expression is an lvalue of class type, it must have a volatile copy constructor to initialize the temporary object that is the result object of the temporary materialization conversion.
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The expression is evaluated and its result (if any) is discarded.