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SERVER-28890 Workaround for upcoming /Zc:ternary

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This workaround allows building MongoDB under Microsoft's upcoming
compiler, with the `/Zc:ternary` option enabled.  This option would
be implied by the `/permissive-` option.  The code fails to compile
under this mode when using Microsoft's existing `std::list`
implementation.  This explicit cast can be removed once Microsoft's
`std::list` stops using inheritance between const and modifiable
iterators.

The issue arises because the `?:` ternary operator requires an
impossible conversion from a `const iterator &` to a
`const_iterator &&` or a `const_iterator &`.  This occurs because
Microsoft's `std::list< ... >::iterator` inherits from
`std::list< ... >::const_iterator`.  Were the `const iterator &`
non-constant (`iterator &`), this would work.  Additionally, were the
`const_iterator &` constant (`const const_iterator &`), this would
also work.

Closes #1147
This commit is contained in:
Yuriy Solodkyy 2017-04-20 12:47:35 -07:00 committed by ADAM David Alan Martin
parent 57d1ad19ed
commit 918ac217df
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1 changed files with 4 additions and 1 deletions
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5
src/mongo/util/lru_cache.h
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@ -131,7 +131,10 @@ public:
*/
const_iterator cfind(const K& key) const {
auto it = this->_map.find(key);
return (it == this->_map.end()) ? this->end() : it->second;
// TODO(SERVER-28890): Remove the function-style cast when MSVC's
// `std::list< ... >::iterator` implementation doesn't conflict with their `/Zc:ternary`
// flag support .
return (it == this->_map.end()) ? this->end() : const_iterator(it->second);
}
/**