[6.2][Concurrency] Forego Sendable checking if conversion doesn't change t…

lib/Sema/TypeCheckConcurrency.cpp

@@ -2865,6 +2865,14 @@ namespace {
               break;
 
             case FunctionTypeIsolation::Kind::GlobalActor:
+              // If the isolation is the same it means that conversion
+              // covers loss of `@Sendable` or some other attribute and
+              // we don't need Sendable checking because there is no
+              // boundary crossing here.
+              if (fromIsolation.getGlobalActorType()->isEqual(
+                      toIsolation.getGlobalActorType()))
+                break;
+
               diagnoseNonSendableParametersAndResult(
                   toFnType, version::Version::getFutureMajorLanguageVersion());
               break;

test/Concurrency/dynamic_checks_for_func_refs_in_preconcurrency_apis.swift

@@ -42,6 +42,10 @@ public struct S<T> {
   }
 }
 
+public class NS {}
+
+public func testNoIsolation(_: @escaping (NS) -> Void) {}
+
 //--- Client.swift
 import API
 
@@ -164,3 +168,14 @@ func testMembers<X>(v: X, s: S<X>, fn: @escaping @MainActor (X) -> Int) {
 // CHECK-NEXT: [[EXEC:%.*]] = extract_executor [[MAIN_ACTOR_ACCESS]] : $MainActor
 // CHECK: [[CHECK_EXEC_REF:%.*]] = function_ref @$ss22_checkExpectedExecutor14_filenameStart01_D6Length01_D7IsASCII5_line9_executoryBp_BwBi1_BwBetF
 // CHECK-NEXT: {{.*}} = apply [[CHECK_EXEC_REF]]({{.*}}, [[EXEC]])
+
+@MainActor
+class Test {
+  @Sendable func compute(_: NS) {}
+  // expected-warning@-1 {{main actor-isolated synchronous instance method 'compute' cannot be marked as '@Sendable'; this is an error in the Swift 6 language mode}}
+
+  func test() {
+    // Triggers loss of @Sendable and actor isolation erasure at the same time
+    testNoIsolation(compute)
+  }
+}