AST: Make HasSelfOrAssociatedTypeRequirementsRequest evaluate to false on cycle

Because we will have considered all the protocols in a cyclic hierarchy
by the time the cycle is hit.

Author: AnthonyLatsis

lib/AST/Decl.cpp

@@ -6707,10 +6707,14 @@ bool ProtocolDecl::existentialConformsToSelf() const {
 }
 
 bool ProtocolDecl::hasSelfOrAssociatedTypeRequirements() const {
+  // Because we will have considered all the protocols in a cyclic hierarchy by
+  // the time the cycle is hit.
+  const bool resultForCycle = false;
+
   return evaluateOrDefault(getASTContext().evaluator,
                            HasSelfOrAssociatedTypeRequirementsRequest{
                                const_cast<ProtocolDecl *>(this)},
-                           true);
+                           resultForCycle);
 }
 
 bool ProtocolDecl::existentialRequiresAny() const {

test/type/explicit_existential_cyclic_protocol.swift

@@ -0,0 +1,35 @@
+// RUN: %target-typecheck-verify-swift -swift-version 5 -verify-additional-prefix no-explicit-any-
+// RUN: %target-typecheck-verify-swift -swift-version 5 -enable-upcoming-feature ExistentialAny -verify-additional-prefix explicit-any-
+
+// REQUIRES: swift_feature_ExistentialAny
+
+// 'HasSelfOrAssociatedTypeRequirementsRequest' should evaluate to false in
+// the event of a cycle because we will have considered all the protocols in a
+// cyclic hierarchy by the time the cycle is hit.
+do {
+  do {
+    protocol P1 : P2 {}
+    // expected-no-explicit-any-note@-1 2 {{protocol 'P1' declared here}}
+    // expected-explicit-any-note@-2 1 {{protocol 'P1' declared here}}
+    protocol P2 : P1 {}
+    // expected-no-explicit-any-error@-1 2 {{protocol 'P2' refines itself}}
+    // expected-explicit-any-error@-2 1 {{protocol 'P2' refines itself}}
+
+    // Diagnosed only with the feature enabled, as a protocol without
+    // "HasSelfOrAssociatedTypeRequirements" should.
+    let _: P2
+    // expected-explicit-any-error@-1 {{use of protocol 'P2' as a type must be written 'any P2'}}
+  }
+  do {
+    protocol P0 { associatedtype A }
+    protocol P1 : P2, P0 {}
+    // expected-no-explicit-any-note@-1 2 {{protocol 'P1' declared here}}
+    // expected-explicit-any-note@-2 1 {{protocol 'P1' declared here}}
+    protocol P2 : P1 {}
+    // expected-no-explicit-any-error@-1 2 {{protocol 'P2' refines itself}}
+    // expected-explicit-any-error@-2 1 {{protocol 'P2' refines itself}}
+
+    let _: P2
+    // expected-error@-1 {{use of protocol 'P2' as a type must be written 'any P2'}}
+  }
+}