SERVER-62498: non-const unique_function should not convert to std::function e…
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…ven after LWG-2774 [LWG-2774](https://wg21.link/lwg2774) changes `std::function`'s converting constructor to accept its argument by forwarding reference. With that change implemented, that constructor becomes a better match for non-`const` `unique_function` arguments than `unique_function`'s `const`-qualified deleted conversion operator template to `std::function`. As a result, `std::is_convertible_v<unique_function<T>, std::function<T>>` changes from `false` to `true` when Standard Libraries implement LWG-2774. (MSVC recently implemented this LWG issue in microsoft/STL#2098 and noticed the `unique_function` test failing as a result.) I believe the fix is to add another deleted conversion operator template that is not `const`-qualified to `unique_function`. This makes the test pass locally, as well as correcting a simplified test case with GCC trunk (microsoft/STL#2098) which I believe also implements LWG-2774.
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Hi @CaseyCarter,
Thanks! |
Contributor
Author
Done!
Also done: https://jira.mongodb.org/browse/SERVER-62498. Do I need to link the ticket and PR somehow? I did add a link to the ticket pointing here. |
Schubes
changed the title
const unique_function should not convert to std::function e…const unique_function should not convert to std::function e…
BillyDonahue
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Jan 11, 2022
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…ven after LWG-2774
LWG-2774 changes
std::function's converting constructor to accept its argument by forwarding reference. With that change implemented, that constructor becomes a better match for non-constunique_functionarguments thanunique_function'sconst-qualified deleted conversion operator template tostd::function. As a result,std::is_convertible_v<unique_function<T>, std::function<T>>changes fromfalsetotruewhen Standard Libraries implement LWG-2774. (MSVC recently implemented this LWG issue in microsoft/STL#2098 and noticed theunique_functiontest failing as a result.)I believe the fix is to add another deleted conversion operator template that is not
const-qualified tounique_function. This makes the test pass for me locally with the updated STL.