A declaration TypeScript *computes* and F# cannot reproduce: a mapped type, a conditional or a template literal at an operand the checker could not resolve (§4.10, §4.11). There is no structure to emit - the structure is a function of an argument not yet supplied - so the declaration is erased and keeps only its name and arity, which is enough for uses of it to stay distinct from each other and from `obj`. Its single case is private, so the only way in or out is a cast, which is exactly the guarantee the generator can honestly make.