E4029

This expression has a type which is not a variant.

Erroneous Example

fn main {
  fn f {
    (a, b) => a + b
  }
  let a = f(None)
}

The example above tries to call a function f with a value None, which is a variant of the Option[_] type, but f only accepts a tuple, giving the following error on line 5:

This expression has type (Int, Int), which is a tuple type and not a variant.

Suggestions

Make sure to use a value of the correct type instead:

fn main {
  fn f {
    (a, b) => a + b
  }
  let a = f((3, 4))
}