SERVER-28890 Workaround for upcoming `/Zc:ternary`

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

1 files changed, 4 insertions, 1 deletions

diff --git a/src/mongo/util/lru_cache.h b/src/mongo/util/lru_cache.h
index 5feea1a0bc3..b59f66e262d 100644
--- a/src/mongo/util/lru_cache.h
+++ b/src/mongo/util/lru_cache.h
@@ -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);
     }
 
     /**