Some expressions implicitly convert types, but this behavior is not always consistent or clearly documented.
$project should have numeric type coercion operators
need a policy for incompatible types in operators in aggregation expressions
Type casting operator for aggregation (and query)
$type projection operator (cast types within aggregation pipelines).
$avg is calculated using integer math for integers even though result value is of type double
dividing two integers always produces a double
consider expanding to wider data type on math overflow
Extend document expression language in aggregation to support advanced document filtering.
aggregation $concat expression is inconsistent in type checking with all other string expressions