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| <?xml version='1.0' encoding='utf-8' standalone='no'?> | ||
| <!DOCTYPE issue SYSTEM "lwg-issue.dtd"> | ||
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| <issue num="4015" status="New"> | ||
| <title>LWG 3973 broke <code>const</code> overloads of <code>std::optional</code> monadic operations</title> | ||
| <section><sref ref="[optional.monadic]"/></section> | ||
| <submitter>Jonathan Wakely</submitter> | ||
| <date>24 Nov 2023</date> | ||
| <priority>99</priority> | ||
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| <discussion> | ||
| <p> | ||
| The resolution of LWG <iref ref="3973"/> (adopted in Kona) changed all | ||
| occurrences of <code>value()</code> to <code>*val</code>. | ||
| The intention was not to change the meaning, just avoid the non-freestanding | ||
| <code>value()</code> function, and avoid ADL that would be caused by using | ||
| <code>**this</code>. | ||
| However, in the <code>const</code> overloads such as | ||
| <code>and_then(F&&) const</code> the type of <code>value()</code> | ||
| was <code>const T&</code>, but the type of <code>*val</code> is always | ||
| <code>T&</code>. This implies that the const overloads invoke the callable | ||
| with a non-const argument, which is incorrect (and would be undefined | ||
| behaviour for a <code>const std::optional<T></code>). | ||
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| </p> | ||
| <p> | ||
| On the LWG reflector it was suggested that we should rewrite the specification | ||
| of <code>std::optional</code> to stop using an exposition-only data member | ||
| of type <code>T*</code>. No such member ever exists in real implemetations, | ||
| so it is misleading and leads to specification bugs of this sort. | ||
| </p> | ||
| <p> | ||
| Change the class definition in <sref ref="[optional.optional.general]"/> | ||
| to use a union, and update every use of <code>val</code> accordingly | ||
| throughout <sref ref="[optional.optional]"/>. | ||
| For consistency with <sref ref="[expected.object.general]"/> we might | ||
| also want to introduce a <code>bool has_val</code> member and refer to | ||
| that in the specification. | ||
| </p> | ||
| <blockquote> | ||
| <pre><code> | ||
| private: | ||
| <del>T *val; <em>// exposition only</em></del> | ||
| <ins>bool has_val; <em>// exposition only</em></ins> | ||
| <ins>union {</ins> | ||
| <ins>T val; <em>// exposition only</em></ins> | ||
| <ins>};</ins> | ||
| }; | ||
| </code></pre> | ||
| </blockquote> | ||
| <p>For example, in <sref ref="[optional.mod]"/>:</p> | ||
| <blockquote> | ||
| <p> | ||
| -1- <i>Effects</i>: | ||
| If <code>*this</code> contains a value, calls | ||
| <code>val<del>-></del><ins>.</ins>T::~T()</code> to destroy the contained | ||
| value<ins> and sets <code>has_val</code> to <code>false</code></ins>; | ||
| otherwise no effect. | ||
| </p> | ||
| </blockquote> | ||
| </discussion> | ||
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| <resolution> | ||
| <p> | ||
| </p> | ||
| </resolution> | ||
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| </issue> |