Cast optional bridged closure parameters to an optional function type

[piercifani]

Problem

A bridged function or init that takes an optional closure fails to compile on Android:

public func f(a: (@MainActor (Int) -> Void)? = nil) { }

The generated Swift support code casts the optional result of SwiftClosureN.closure(forJavaObject:options:) to a non-optional function type:

// before: error: value of optional type '(@Sendable (Int) -> Void)?' must be unwrapped
let p_0_swift = SwiftClosure1.closure(forJavaObject: p_0, options: []) as @MainActor (Int) -> Void
// after
let p_0_swift = SwiftClosure1.closure(forJavaObject: p_0, options: []) as (@MainActor (Int) -> Void)?

This affects all optional closure parameters, with or without @MainActor.

Fix

When the parameter is optional, cast to the optional function type. Non-optional closures are unchanged (closure(...)! as (P) -> R).

Tests

Added testFunctionOptionalClosureParameter to BridgeToKotlinTests. SkipSyntaxTests pass. Also checked with swiftc -swift-version 6 that the new cast type-checks against the (@Sendable (P0) -> R)? return type of closure(forJavaObject:options:).

🤖 Generated with Claude Code