Sema: Avoid another cycle with generic signatures

We can determine the associated types referenced from a function signature
by canonicalizing the individual parameter and result types, rather than
first canonicalizing the entire function type. Canonicalizing a function
type creates a GenericSignatureBuilder, which can cause cycles.

This fixes some cycles when the requirement machine is enabled.

Author: slavapestov

lib/Sema/TypeCheckProtocol.cpp

@@ -2684,7 +2684,18 @@ ConformanceChecker::getReferencedAssociatedTypes(ValueDecl *req) {
   };
 
   Walker walker(Proto, assocTypes);
-  req->getInterfaceType()->getCanonicalType().walk(walker);
+
+  // This dance below is to avoid calling getCanonicalType() on a
+  // GenericFunctionType, which creates a GenericSignatureBuilder, which
+  // can in turn trigger associated type inference and cause a cycle.
+  auto reqTy = req->getInterfaceType();
+  if (auto *funcTy = reqTy->getAs<GenericFunctionType>()) {
+    for (auto param : funcTy->getParams())
+      param.getPlainType()->getCanonicalType().walk(walker);
+    funcTy->getResult()->getCanonicalType().walk(walker);
+  } else {
+    reqTy->getCanonicalType().walk(walker);
+  }
 
   return assocTypes;
 }