Example:

  fun sum(a: Int, b: Int) = { a + b } // The return type of this function is '() -> Int'.

There are several quick fixes. After applying them:

    fun sum(a: Int, b: Int) = a + b // Remove braces
    fun sum(a: Int, b: Int): () -> Int = { a + b } // Specify return type explicitly
    fun sum(a: Int, b: Int) = run { a + b } // Convert to run { ... }
    fun sum(a: Int, b: Int) = { -> a + b } // Specify explicit lambda signature