Building mongod/mongos on Windows with --opt=off/--dbg=on fails with the following error:
collator_interface_icu.obj : error LNK2019: unresolved external symbol "__declspec(dllimport) public: unsigned char const * __cdecl icu_57::CollationKey::getByteArray(int &)const " (__imp_?getByteArray@CollationKey@icu_57@@QEBAPEBEAEAH@Z) referenced in function "public: virtual class mongo::CollatorInterface::ComparisonKey __cdecl mongo::CollatorInterfaceICU::getComparisonKey(class mongo::StringData)const " (?getComparisonKey@CollatorInterfaceICU@mongo@@UEBA?AVComparisonKey@CollatorInterface@2@VStringData@2@@Z)
The same build succeeds, when --opt=on is used.