Add cross-module tests for default isolation behavior

Author: DougGregor

test/Concurrency/Inputs/implicit_nonisolated_things.swift

@@ -0,0 +1,7 @@
+public protocol ImportedSendableProto: Sendable { }
+
+@MainActor public struct ImportedStruct: ImportedSendableProto { }
+
+public struct ImportedOtherStruct { }
+
+public protocol ImportedP { }

test/Concurrency/assume_mainactor_typechecker_errors.swift

@@ -1,11 +1,15 @@
-// RUN: %target-swift-frontend -swift-version 5 -emit-sil -default-isolation MainActor %s -verify -verify-additional-prefix swift5-
-// RUN: %target-swift-frontend -swift-version 6 -emit-sil -default-isolation MainActor %s -verify -verify-additional-prefix swift6-
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module -swift-version 6 -module-name implicit_nonisolated_things -o %t/implicit_nonisolated_things.swiftmodule %S/Inputs/implicit_nonisolated_things.swift
+// RUN: %target-swift-frontend -I %t -swift-version 5 -emit-sil -default-isolation MainActor %s -verify -verify-additional-prefix swift5-
+// RUN: %target-swift-frontend -I %t  -swift-version 6 -emit-sil -default-isolation MainActor %s -verify -verify-additional-prefix swift6-
 
 // READ THIS! This test is meant to check the specific isolation when
 // `-default-isolation` is set to `MainActor` in combination with validating
 // behavior around explicitly non-Sendable types that trigger type checker
 // specific errors. Please do not put other types of tests in here.
 
+import implicit_nonisolated_things
+
 // Fake Sendable Data
 class SendableData : @unchecked Sendable {}
 // expected-swift5-note@-1 {{calls to initializer 'init()' from outside of its actor context are implicitly asynchronous}}
@@ -82,7 +86,7 @@ func testTaskDetached() async {
 
 // @MainActor
 extension Int {
-  func memberOfInt() { } // expected-note 2{{calls to instance method 'memberOfInt()' from outside of its actor context are implicitly asynchronous}}
+  func memberOfInt() { } // expected-note 3{{calls to instance method 'memberOfInt()' from outside of its actor context are implicitly asynchronous}}
 }
 
 nonisolated func testMemberOfInt(i: Int) {
@@ -127,6 +131,34 @@ struct MyP: P {
   }
 }
 
+// Above tests for imported types
+extension ImportedStruct: @retroactive CustomStringConvertible {
+  public var description: String {
+    17.memberOfInt() // okay, on main actor
+    return "hello"
+  }
+}
+
+extension ImportedOtherStruct {
+  func f() {
+    17.memberOfInt() // okay, on main actor
+  }
+}
+
+nonisolated
+extension ImportedOtherStruct {
+  func g() {
+    17.memberOfInt() // expected-swift5-warning{{call to main actor-isolated instance method 'memberOfInt()' in a synchronous nonisolated context}}
+  // expected-swift6-error@-1{{call to main actor-isolated instance method 'memberOfInt()' in a synchronous nonisolated context}}
+  }
+}
+
+struct MyImportedP: ImportedP {
+  func g() {
+    17.memberOfInt() // okay, on main actor
+  }
+}
+
 // https://github.com/swiftlang/swift/issues/82168 -
 nonisolated protocol OtherP {
   associatedtype AT