investigate downcast type safety

[mmoskal]

Following scenarios need investigating:

• explicit downcast as in foo as Derived (or <Derived>foo)
• implicit downcast due to bivariant function types: TS FAQ
• covariant arrays
• indexing types without explicit indexer

I think we could introduce some runtime checks for the above - upon explicit casting, entry into a lambda function, and upon assigning to arrays.

We cannot really support union types (there is currently no way to distinguish at runtime between a number and a reference for example). Intersection types are I think only compile-time feature.

[mmoskal]

CC @tballmsft

[tballmsft]

forget about union/intersection types, say I.

[tballmsft]

New type checker disallows downcasts, bivariance. Closing for now

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.