The expression.h registration macros could be refactored to use REGISTER_EXPRESSION_CONDITIONALLY rather than all calling MONGO_INITIALIZER_GENERAL and registerExpression .

Currently if we try to refactor them this results in a compilation error on the Windows variants (see BF-25979)

src\mongo\db\pipeline\expression.cpp(571,1): error C2143: syntax error: missing ';' before '&&'
 REGISTER_STABLE_EXPRESSION(and, ExpressionAnd::parse);

This seems to be because the word 'and' in the Windows preprocessor is a macro for && : https://gcc.gnu.org/onlinedocs/cpp/C_002b_002b-Named-Operators.html#C_002b_002b-Named-Operators

We should look into if it's possible to get around this