Directly detect the cyclic case while computing conditional requirement

The general class of cycle here is when validation asks for the generic signature which triggers requirement checking which forces us to ask for the generic signature of the extension again.  We should look into a more principled solution.

See rdar://55263708

Author: CodaFi

include/swift/AST/Decl.h

@@ -1536,6 +1536,7 @@ class GenericContext : private _GenericContext, public DeclContext {
   /// \endcode
   bool isGeneric() const { return getGenericParams() != nullptr; }
   bool hasComputedGenericSignature() const;
+  bool isComputingGenericSignature() const;
 
   /// Retrieve the trailing where clause for this extension, if any.
   TrailingWhereClause *getTrailingWhereClause() const {

lib/AST/Decl.cpp

@@ -859,6 +859,11 @@ GenericParamList *GenericContext::getGenericParams() const {
 bool GenericContext::hasComputedGenericSignature() const {
   return GenericSigAndBit.getInt();
 }
+      
+bool GenericContext::isComputingGenericSignature() const {
+  return getASTContext().evaluator.hasActiveRequest(
+                 GenericSignatureRequest{const_cast<GenericContext*>(this)});
+}
 
 GenericSignature *GenericContext::getGenericSignature() const {
   return evaluateOrDefault(

lib/AST/ProtocolConformance.cpp

@@ -531,7 +531,7 @@ void NormalProtocolConformance::differenceAndStoreConditionalRequirements()
     assert(CRState == ConditionalRequirementsState::Computing);
     CRState = ConditionalRequirementsState::Uncomputed;
   };
-
+  
   auto &ctxt = getProtocol()->getASTContext();
   auto DC = getDeclContext();
   // A non-extension conformance won't have conditional requirements.
@@ -550,30 +550,24 @@ void NormalProtocolConformance::differenceAndStoreConditionalRequirements()
     return;
   }
 
-  // The type is generic, but the extension doesn't have a signature yet, so
-  // we might be in a recursive validation situation.
-  if (!ext->hasComputedGenericSignature()) {
-    // If the extension is invalid, it won't ever get a signature, so we
-    // "succeed" with an empty result instead.
-    if (ext->isInvalid()) {
-      success({});
-      return;
-    }
+  // If the extension is invalid, it won't ever get a signature, so we
+  // "succeed" with an empty result instead.
+  if (ext->isInvalid()) {
+    success({});
+    return;
+  }
 
-    // Otherwise we'll try again later.
+  // Recursively validating the signature comes up frequently as expanding
+  // conformance requirements might re-enter this method.  We can at least catch
+  // this and come back to these requirements later.
+  //
+  // FIXME: In the long run, break this cycle in a more principled way.
+  if (ext->isComputingGenericSignature()) {
     failure();
     return;
   }
-  
-  // FIXME: All of this will be removed when validateExtension goes away.
-  auto extensionSig = ext->getGenericSignature();
-  if (!extensionSig) {
-    if (auto lazyResolver = ctxt.getLazyResolver()) {
-      lazyResolver->resolveExtension(ext);
-      extensionSig = ext->getGenericSignature();
-    }
-  }
 
+  auto extensionSig = ext->getGenericSignature();
   auto canExtensionSig = extensionSig->getCanonicalSignature();
   auto canTypeSig = typeSig->getCanonicalSignature();
   if (canTypeSig == canExtensionSig) {