Allow upcast in type test pattern (inside computation expression)
See this SO subject along with Tomas response ; that convey pretty well what i mean (as my english is rubbish).
See comment above
Don Syme, F# Language Evolution
Don Syme commented
Perhaps I don't fully understand the suggestion. However essentially we decided against this in the design of computation expressions, instead the recommendation is to use "match" if doing a runtime type test, and ":>" if doing a static cast.