Clean up of discarded-value expressions #3909
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jensmaurer
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Apr 10, 2020
jensmaurer
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Apr 13, 2020
zygoloid
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Sep 18, 2020
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| \begin{note} | ||
| If the 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 lvalue-to-rvalue | ||
| conversion. | ||
| \end{note} |
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This note is quite distant from "the expression" that it's talking about, which is the original discarded-value expression prior to any conversions. Can we get it a bit closer to the wording above that talks about introducing an lvalue-to-rvalue conversion?
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But the temporary materialization conversion is the one that actually creates the object (and needs the volatile copy constructor).
Suggestion: Say "If the original expression" and replace "lvalue-to-rvalue conversion" with "temp. mat. conv." and move "The expression is evaluated and its result (if any) is discarded." to after the note.
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This fixes a few issues with discarded-value expressions:
The current wording in [expr.prop] states that the temporary materialization conversion is applied to prvalues of
voidtype, which makes discarded-value expressions ofvoidtype ill-formed (temporary materialization conversion requires a complete type). This fixes that, and ensures that all discarded-value expressions are evaluated (with the exception of unevaluated operands, of course)In [expr.static.cast], grammatical term expression is used where "operand" is much clearer, and more consistent with the rest of the wording. Additionally, specifying that the operand of the cast becomes a discarded-value expression is much clearer than saying "the expression", since it could be interpreted as saying that the whole expression is a discarded value expression (it shouldn't, since the operand itself needs to be temporarily materialized and evaluated). A note is also added to clarify that the conversion will be an expression of type
voidwith no result.A
returnstatement can sometimes have an operand of typevoid, in which case it should be a discarded-value expression, but this is not specified.As for the editorial nature of this, the intent here is pretty clear :)
Will rename commit after edits :)