If the argument is a CommaExp, set a flag to prevent deprecation messages
Temporary variable to enable / disable deprecation of comma expression depending on the context. Since most constructor calls are rewritting, the only place where false will be passed will be from the parser.
This is needed because AssignExp rewrites CommaExp, hence it needs to trigger the deprecation.
The types for a binary expression are incompatible. Print error message.
Mark the operands as will never be dereferenced, which is useful info for @safe checks. Do before semantic() on operands rewrites them.