Investigate why partial type definition being coalesced to non partial isn't a TypeScript compiler

[phillco]

NTS: The actual type of the interface is requirements?: Partial<ScopeIteratorRequirements>. This should really be a tsc compile error but it wasn't. We therefore started to read properties on hints when some of those properties could be undefined which threw an exception

Originally posted by @phillco in #2739 (comment)

[phillco]

Just flagging for passive followup later

[AndreasArvidsson]

I created the smallest possible example. No warnings from typescript.

#2745