Non-MMAP V1 implementations of the writingPtr method all report warnings because of the usage of invariant:
e:\workspace\mongo\src\mongo\db\storage\ephemeral_for_test\ephemeral_for_test_recovery_unit.h(60) : warning C4715: 'mongo::EphemeralForTestRecoveryUnit::writingPtr' : not all control paths return a value ... e:\workspace\mongo\src\mongo\db\storage\wiredtiger\wiredtiger_recovery_unit.h(73) : warning C4715: 'mongo::WiredTigerRecoveryUnit::writingPtr' : not all control paths return a value