Sema: Fix request cycle with isolation inference

Fixes rdar://129024926.

Author: slavapestov

lib/Sema/TypeCheckConcurrency.cpp

@@ -4456,6 +4456,11 @@ getIsolationFromAttributes(const Decl *decl, bool shouldDiagnose = true,
 /// Infer isolation from witnessed protocol requirements.
 static std::optional<ActorIsolation>
 getIsolationFromWitnessedRequirements(ValueDecl *value) {
+  // Associated types cannot have isolation, so there's no such inference for
+  // type witnesses.
+  if (isa<TypeDecl>(value))
+    return std::nullopt;
+
   auto dc = value->getDeclContext();
   auto idc = dyn_cast_or_null<IterableDeclContext>(dc->getAsDecl());
   if (!idc)

test/Concurrency/sendable_checking.swift

@@ -481,3 +481,13 @@ func checkOpaqueType() -> some Sendable {
   UnavailableSendable()
   // expected-warning@-1 {{conformance of 'UnavailableSendable' to 'Sendable' is unavailable; this is an error in the Swift 6 language mode}}
 }
+
+// rdar://129024926
+
+@available(SwiftStdlib 5.1, *)
+@MainActor class MainActorSuper<T: Sendable> {}
+
+@available(SwiftStdlib 5.1, *)
+class MainActorSub: MainActorSuper<MainActorSub.Nested> {
+  struct Nested {}  // no cycle
+}