Fix issue where uncurried type internals leak in type error.

[cristianoc]

Fixes #6263

  We've found a bug for you!
  tst.res:4:13-22

  2 │ 
  3 │ let foo = (x: array<_>) => ()
  4 │ let x = foo(a => a + 1)
  5 │ 

  This has type: _ => _
  Somewhere wanted: array<'b>