[Concurrency] Global actor isolated conformances are only allowed to override nonisolated.

Follow-up for #79893.

More than one global actor isolated conformance at any level
creates a clash and conforming type should be inferred as `nonisolated`.

Resolves: rdar://154202375
(cherry picked from commit 2e1fe44)

Author: xedin

lib/Sema/TypeCheckConcurrency.cpp

@@ -5408,13 +5408,12 @@ getIsolationFromConformances(NominalTypeDecl *nominal) {
 
     case ActorIsolation::GlobalActor:
       // If we encountered an explicit globally isolated conformance, allow it
-      // to override the nonisolated isolation kind.
+      // to override the _nonisolated_ isolation.
       if (!foundIsolation ||
-          conformance->getSourceKind() == ConformanceEntryKind::Explicit) {
-        foundIsolation = {
-          protoIsolation,
-          IsolationSource(proto, IsolationSource::Conformance)
-        };
+          (foundIsolation->isolation.isNonisolated() &&
+           conformance->getSourceKind() == ConformanceEntryKind::Explicit)) {
+        foundIsolation = {protoIsolation,
+                          IsolationSource(proto, IsolationSource::Conformance)};
         continue;
       }
 

test/Concurrency/nonisolated_rules.swift

@@ -5,6 +5,19 @@
 @MainActor
 protocol GloballyIsolated {}
 
+@globalActor
+actor Test {
+  static let shared: Test = Test()
+}
+
+@Test
+protocol TestIsolatedProto {
+}
+
+@Test
+protocol RedeclaredIsolationProto : GloballyIsolated {
+}
+
 // expected-note@+1 {{class 'NonSendable' does not conform to the 'Sendable' protocol}}
 class NonSendable {}
 
@@ -312,3 +325,25 @@ func rdar147965036() {
     // expected-error@-2 {{'nonisolated' is not supported on a closure}}
   }
 }
+
+// Test that clash in isolation from protocols results in nonisolated conforming type
+func testProtocolIsolationClash() {
+  struct A: GloballyIsolated, TestIsolatedProto {
+  }
+
+  struct B: RedeclaredIsolationProto {
+  }
+
+  struct C: GloballyIsolated, TestIsolatedProto, WhyNot {
+  }
+
+  struct D: WhyNot, GloballyIsolated, TestIsolatedProto {
+  }
+
+  nonisolated func test() {
+    _ = A() // Ok
+    _ = B() // Ok
+    _ = C() // Ok
+    _ = D() // Ok
+  }
+}