In src/mongo/client/connpool.h, class PoolForHost has a default constructor PoolForHost::PoolForHost() which does not initialize the _socketTimeoutSecs member (a raw double), which has no default member initializer, so a default-constructed instance holds an indeterminate _socketTimeoutSecs. This member is used in PoolForHost::clear() when logging dropped connections via _makeDuration(_socketTimeoutSecs), and reading the uninitialized value constitutes undefined behavior, potentially causing unexpected behavior or crashes if _socketTimeoutSecs is not set before clear() is called.
Found by static analysis (Svace, UNINIT.CTOR): "Constructor declared at connpool.cpp:104 may not initialize class members of 'mongo::PoolForHost'. Following members aren't initialized: _socketTimeoutSecs."
The fix is to add a default member initializer (double _socketTimeoutSecs = 0.0 or initialize it explicitly in the constructor initializer list.