Sema: If the structural occurs check skips a witness, treat it as tautological

Fixes rdar://problem/122584912.

Author: slavapestov

lib/Sema/AssociatedTypeInference.cpp

@@ -1493,6 +1493,10 @@ AssociatedTypeInference::getPotentialTypeWitnessesFromRequirement(
 
     if (cycleCheck.checkForPotentialCycle(witness)) {
       LLVM_DEBUG(llvm::dbgs() << "Skipping witness to avoid request cycle\n");
+
+      // We must consider the possibility that none of the witnesses for this
+      // requirement can be chosen.
+      hadTautologicalWitness = true;
       continue;
     }
 

test/Distributed/actor_protocols.swift

@@ -67,8 +67,6 @@ actor A2: DistributedActor {
 final class DA2: DistributedActor {
 // expected-error@-1{{non-actor type 'DA2' cannot conform to the 'AnyActor' protocol}}
 // expected-error@-2{{non-distributed actor type 'DA2' cannot conform to the 'DistributedActor' protocol}}
-// expected-error@-3{{'DistributedActor' requires the types 'ObjectIdentifier' and 'FakeActorSystem.ActorID' (aka 'ActorAddress') be equivalent}}
-// expected-note@-4{{requirement specified as 'Self.ID' == 'Self.ActorSystem.ActorID' [with Self = DA2]}}
   nonisolated var id: ID {
     fatalError()
   }

test/decl/protocol/req/rdar122584912.swift

@@ -0,0 +1,27 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-associated-type-inference
+// RUN: %target-typecheck-verify-swift -disable-experimental-associated-type-inference
+
+public struct DefaultA {}
+
+public protocol P {
+    associatedtype A = DefaultA
+    associatedtype B
+
+    static func foo(_: A, _: B)
+    static func bar(_: B)
+}
+
+extension P {
+    public static func foo(_: A, _: B) where B == Void {}
+}
+
+// The only way C can conform to P is with 'A := DefaultA' and 'B := String'.
+// We skip C.foo() because it contains 'A', so we must not infer 'B := Void'
+// from P.foo() above.
+class C: P {
+    public static func foo(_: A, _: String) {}
+    public static func bar(_: String) {}
+}
+
+let x: DefaultA.Type = C.A.self
+let y: String.Type = C.B.self