This error occurs when an `if` expression without an `else` block is used in a
context where a type other than `()` is expected, for example a `let`
expression:

```compile_fail,E0317
fn main() {
    let x = 5;
    let a = if x == 5 { 1 };
}
```

An `if` expression without an `else` block has the type `()`, so this is a type
error. To resolve it, add an `else` block having the same type as the `if`
block.