Experimenting with Playground. The following compiles with an error:

type t = { optValue?: int, } let x = Some(3) let s1 = {optValue: ?(x->Belt.Option.map(a => a+1))} // <-- Type error let s2 = {optValue: ?(Belt.Option.map(x, a => a+1))}

Type Errors [E] Line 6, column 22: This has type: option<int> Somewhere wanted: int 

Screenshot:

So, the pipe expression highlighted has indeed type option<int> but the compiler wants a plain int for some reason.

Interesting, the same expression without the pipe compiles just fine.