Sema: Less eager computation of protocol extension signatures in associated type inference

We only need the generic signature if the extended nominal is
concrete. In the protocol extension case, we're using
getSelfBoundsFromWhereClause(), which only needs to perform
name lookup.

Author: slavapestov

lib/Sema/TypeCheckProtocolInference.cpp

@@ -207,22 +207,22 @@ AssociatedTypeInference::inferTypeWitnessesViaValueWitnesses(
     if (extendedNominal == nullptr)
       return true;
 
-    // FIXME: The extension may not have a generic signature set up yet as
-    // resolving signatures may trigger associated type inference.  This cycle
-    // is now detectable and we should look into untangling it
-    // - see rdar://55263708
-    if (!extension->hasComputedGenericSignature())
-      return true;
-
-    // Retrieve the generic signature of the extension.
-    const auto extensionSig = extension->getGenericSignature();
-
     auto *proto = dyn_cast<ProtocolDecl>(extendedNominal);
 
     // If the extension is bound to the nominal the conformance is
     // declared on, it is viable for inference when its conditional
     // requirements are satisfied by those of the conformance context.
     if (!proto) {
+      // FIXME: The extension may not have a generic signature set up yet as
+      // resolving signatures may trigger associated type inference.  This cycle
+      // is now detectable and we should look into untangling it
+      // - see rdar://55263708
+      if (!extension->hasComputedGenericSignature())
+        return true;
+
+      // Retrieve the generic signature of the extension.
+      const auto extensionSig = extension->getGenericSignature();
+
       // Extensions of non-generic nominals are always viable for inference.
       if (!extensionSig)
         return true;
@@ -261,9 +261,6 @@ AssociatedTypeInference::inferTypeWitnessesViaValueWitnesses(
     return true;
   };
 
-  auto typeInContext =
-    conformance->getDeclContext()->mapTypeIntoContext(conformance->getType());
-
   for (auto witness :
        checker.lookupValueWitnesses(req, /*ignoringNames=*/nullptr)) {
     LLVM_DEBUG(llvm::dbgs() << "Inferring associated types from decl:\n";
@@ -319,6 +316,10 @@ AssociatedTypeInference::inferTypeWitnessesViaValueWitnesses(
           if (!associatedTypesAreSameEquivalenceClass(dmt->getAssocType(),
                                                       result.first))
             return false;
+
+          auto typeInContext =
+            conformance->getDeclContext()->mapTypeIntoContext(conformance->getType());
+
           if (!dmt->getBase()->isEqual(typeInContext))
             return false;