Cannot determine expression for data requirement

[brynrhodes]

When attempting to determine data requirements for a specific library, the following error is encountered:

"Cannot determine expression for data requirement because the source is not an AliasedQuerySource or LetClause"

This can occur when the primary source for the query is a property, so the "retrieve" is a manufactured retrieve to represent the primary source. When this occurs, there is no data type on the retrieve, and the resulting attempt to access the type through the expression of the source fails.