Define a new IDL struct that includes an field of variant type, such as structs: CreateCommandRequest: strict: true fields: clusteredIndex: type: variant: [safeBool, ClusteredIndexSpec] optional: true unstable: false Compile the server code Expected outcome : the IDL generator should produce valid gen.h/cpp files Observed outcome : the code produced by the IDL generator cannot be compiled Sample from the generated code: void CreateCommand::parseProtected(const IDLParserErrorContext& ctxt, const BSONObj& bsonObject) { ... else if (fieldName == kClusteredIndexFieldName) { if (MONGO_unlikely(usedFields[kClusteredIndexBit])) { ctxt.throwDuplicateField(element); } usedFields.set(kClusteredIndexBit); const BSONType variantType = element.type(); switch (variantType) { ... case NumberDouble: { _createCommandRequest.setClusteredIndex(boost::optional<stdx::variant<bool, mongo::ClusteredIndexSpec>>(element.trueValue())); break; } case Object: _createCommandRequest.setClusteredIndex(mongo::ClusteredIndexSpec::parse(ctxt, element.Obj())); // <-- THE OFFENDING LINE break; } } Compilation error: [210/983 ( 21%) 8.508s] Compiled build/optdebug/mongo/db/commands/create_gen.dyn.o FAILED: build/optdebug/mongo/db/commands/create_gen.dyn.o export CCACHE_NOCPP2='1';export CCACHE_PREFIX='/home/ubuntu/mongo/build/scons/icecream/optdebug/run-icecc.sh';export ICECC_CLANG_REMOTE_CPP='1';export PATH='/opt/mongodbtoolchain/v3/bin:/usr/local/bin:/opt/bin:/bin:/usr/bin';/usr/bin/ccache /opt/mongodbtoolchain/v3/bin/clang++ @build/optdebug/mongo/db/commands/create_gen.dyn.o.rsp build/optdebug/mongo/db/commands/create_gen.cpp:1083:57: error: no viable conversion from 'mongo::ClusteredIndexSpec' to 'boost::optional<stdx::variant<bool, mongo::ClusteredIndexSpec> >' _createCommandRequest.setClusteredIndex(mongo::ClusteredIndexSpec::parse(ctxt, element.Obj())); ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ src/third_party/boost/boost/optional/optional.hpp:934:5: note: candidate constructor not viable: no known conversion from 'mongo::ClusteredIndexSpec' to 'boost::none_t' for 1st argument optional( none_t none_ ) BOOST_NOEXCEPT : base(none_) {} ^ src/third_party/boost/boost/optional/optional.hpp:938:5: note: candidate constructor not viable: no known conversion from 'mongo::ClusteredIndexSpec' to 'boost::optional<mpark::variant<bool, mongo::ClusteredIndexSpec> >::argument_type' (aka 'const mpark::variant<bool, mongo::ClusteredIndexSpec> &') for 1st argument optional ( argument_type val ) : base(optional_detail::init_value_tag(), val) {} ^ src/third_party/boost/boost/optional/optional.hpp:943:5: note: candidate constructor not viable: no known conversion from 'mongo::ClusteredIndexSpec' to 'boost::optional<mpark::variant<bool, mongo::ClusteredIndexSpec> >::rval_reference_type' (aka 'mpark::variant<bool, mongo::ClusteredIndexSpec> &&') for 1st argument optional ( rval_reference_type val ) : base(optional_detail::init_value_tag(), boost::forward<T>(val)) ^ src/third_party/boost/boost/optional/optional.hpp:1023:5: note: candidate constructor not viable: no known conversion from 'mongo::ClusteredIndexSpec' to 'const boost::optional<mpark::variant<bool, mongo::ClusteredIndexSpec> > &' for 1st argument optional ( optional const& ) = default; ^ src/third_party/boost/boost/optional/optional.hpp:1033:5: note: candidate constructor not viable: no known conversion from 'mongo::ClusteredIndexSpec' to 'boost::optional<mpark::variant<bool, mongo::ClusteredIndexSpec> > &&' for 1st argument optional ( optional && ) = default; ^ build/optdebug/mongo/db/commands/create_gen.h:188:92: note: passing argument to parameter 'value' here void setClusteredIndex(boost::optional<stdx::variant<bool, mongo::ClusteredIndexSpec>> value) { _clusteredIndex = std::move(value); } ^ build/optdebug/mongo/db/commands/create_gen.cpp:1493:57: error: no viable conversion from 'mongo::ClusteredIndexSpec' to 'boost::optional<stdx::variant<bool, mongo::ClusteredIndexSpec> >' _createCommandRequest.setClusteredIndex(mongo::ClusteredIndexSpec::parse(ctxt, element.Obj())); ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ src/third_party/boost/boost/optional/optional.hpp:934:5: note: candidate constructor not viable: no known conversion from 'mongo::ClusteredIndexSpec' to 'boost::none_t' for 1st argument optional( none_t none_ ) BOOST_NOEXCEPT : base(none_) {} ^ src/third_party/boost/boost/optional/optional.hpp:938:5: note: candidate constructor not viable: no known conversion from 'mongo::ClusteredIndexSpec' to 'boost::optional<mpark::variant<bool, mongo::ClusteredIndexSpec> >::argument_type' (aka 'const mpark::variant<bool, mongo::ClusteredIndexSpec> &') for 1st argument optional ( argument_type val ) : base(optional_detail::init_value_tag(), val) {} ^ src/third_party/boost/boost/optional/optional.hpp:943:5: note: candidate constructor not viable: no known conversion from 'mongo::ClusteredIndexSpec' to 'boost::optional<mpark::variant<bool, mongo::ClusteredIndexSpec> >::rval_reference_type' (aka 'mpark::variant<bool, mongo::ClusteredIndexSpec> &&') for 1st argument optional ( rval_reference_type val ) : base(optional_detail::init_value_tag(), boost::forward<T>(val)) ^ src/third_party/boost/boost/optional/optional.hpp:1023:5: note: candidate constructor not viable: no known conversion from 'mongo::ClusteredIndexSpec' to 'const boost::optional<mpark::variant<bool, mongo::ClusteredIndexSpec> > &' for 1st argument optional ( optional const& ) = default; ^ src/third_party/boost/boost/optional/optional.hpp:1033:5: note: candidate constructor not viable: no known conversion from 'mongo::ClusteredIndexSpec' to 'boost::optional<mpark::variant<bool, mongo::ClusteredIndexSpec> > &&' for 1st argument optional ( optional && ) = default; ^ build/optdebug/mongo/db/commands/create_gen.h:188:92: note: passing argument to parameter 'value' here void setClusteredIndex(boost::optional<stdx::variant<bool, mongo::ClusteredIndexSpec>> value) { _clusteredIndex = std::move(value); }