[Sema] Fix #84644: Implicit initializers now inherit default actor isolation.

[Harshit7m]

Resolves #84644

Problem:
When a class and its subclass have no explicit initializers, the compiler would emit:

Main actor-isolated initializer 'init()' has different actor isolation from nonisolated overridden declaration

This occurs under Swift 6.2 with default @MainActor isolation enabled. The synthesized default initializer for the superclass was not inheriting the class’s default actor isolation, causing a mismatch when a subclass implicitly overrides it.

Solution:
Ensure that synthesized default initializers inherit the class’s default actor isolation:

if (auto isolation = decl->getDefaultActorIsolation()) {
    ctor->setActorIsolation(isolation);
}

This guarantees compatibility between synthesized initializer of superclass and any of its subclass, even if neither declares an explicit init().

Regression Test:
Added test/concurrency/default_actor_isolation_initializer.swift:

// RUN: %target-typecheck-verify-swift -enable-upcoming-feature GlobalActorIsMainActor -enable-upcoming-feature RegionBasedIsolation
// expected-no-diagnostics

@MainActor
class Thing {}

class SubThing: Thing {}

The test confirms that classes without explicit initializers now compile correctly when default actor isolation is applied.

Impact:

• Fixes the compiler crash/error for subclasses of @MainActor classes with implicit initializers.
• Adds a regression test to prevent future regressions.
• Minimal change in CodeSynthesis.cpp.

[Harshit7m]

Hello @xedin if you get a chance, could you help trigger CI? Thank you!

[xedin]

Sure, I will take a look in a bit and trigger CI if everything looks good. Thank you!

[xedin]

Since the main-actor mode is on by default, this annotation is not needed.

[xedin]

We don't have GlobalActorIsMainActor, it's actually spelled as -default-isolation MainActor, also I don't think you need RegionBasedIsolation here either.

[xedin]

We need to check SILGen output to make sure that initializers are isolated correctly, both initializers should be marked as MainActor in the output. I think you can move this to test/Concurrency/assume_mainactor.swift and add the checks there (it already checks for deinit).

[xedin]

I don't think there is a getDefaultActorIsolation method also I don't think we want a "default" here we want to match isolation of the type, that's the isolation we need to get.