flatMap on UIView.constraints type inference confusion with no return type change in closure

[xwu (Xiaodi Wu)]

This behavior occurs because:

If a closure contains a single expression, Swift will consider its body in addition to its signature and the surrounding context when performing type inference. [...] If a closure body is not a single expression, it will not be considered when inferring the closure type.

The reason for this is as follows:

@jrose

Swift's type inference is currently statement-oriented, so there's no easy way to do this inference. This is at least partly a compilation-time concern: Swift's type system allows many more possible conversions than, say, Haskell or OCaml, so solving the types for an entire multi-statement function is not a trivial problem, possibly not a tractable problem.

See the compiler's educational note for closure type inference for more information on how to address this situation in your code.