Introducing `if statement` `for statement` into swift. Required swift 5.4.

Expressive.swift

public enum Either<L, R> {
    case left(L)
    case right(R)
}

@resultBuilder
public struct Expressive<T> {
    public static func buildBlock<C>() -> C? { nil }
    public static func buildBlock<C>(_ component: C) -> C { component }
    public static func buildEither(first component: T) -> T { component }
    public static func buildEither(second component: T) -> T { component }
    public static func buildOptional<C>(_ component: C?) -> C? { component }
    public static func buildArray<C>(_ components: [C]) -> [C] { components }
    public static func buildEither<F, S>(first component: F) -> Either<F, S> { .left(component) }
    public static func buildEither<F, S>(second component: S) -> Either<F, S> { .right(component) }
}

public func expressive<T>(leastCommon: T.Type = T.self, @Expressive<T> _ f: () -> T) -> T { f() }
public func expressive<T, R>(leastCommon: T.Type = T.self, @Expressive<T> _ f: () -> R) -> R { f() }

ExpressiveTests.swift

import XCTest
@testable import Expressive

protocol Animal {}
struct Cat: Animal {}
struct Dog: Animal {}
struct Bird: Animal {}

final class ExpressiveTests: XCTestCase {
    func testTypeInferences() {
        XCTAssertTrue(
            expressive {
                if true {
                    1
                }
            } is Optional<Int>
        )

        XCTAssertTrue(
            expressive {
                if true {
                    1
                } else {
                    2
                }
            } is Int
        )

        XCTAssertTrue(
            expressive {
                if true {
                    Cat()
                } else {
                    Dog()
                }
            } is Either<Cat, Dog>
        )

        XCTAssertTrue(
            expressive {
                if true {
                    Cat()
                } else {
                    Dog()
                }
            } as Animal is Animal
        )

        XCTAssertTrue(
            expressive {
                for i in 0..<10 {
                    i
                }
            } is [Int]
        )

        XCTAssertTrue(
            expressive {
                for i in 0..<10 {
                    if i % 2 == 0 {
                        Cat()
                    } else {
                        Dog()
                    }
                }
            } is [Either<Cat, Dog>]
        )

        XCTAssertTrue(
            expressive(leastCommon: Animal.self) {
                for i in 0..<10 {
                    if i % 2 == 0 {
                        Cat()
                    } else {
                        Dog()
                    }
                }
            } is [Animal]
        )
    }
}