[NamedLazyMemberLoading] Use per-IDC recursion-breaking flag, not sentinel.

The empty sentinel in the lookup table caused recursion-breaking to bottom
out in slightly different order than the old eager code, making certain
order-sensitive tests fail.

Author: graydon

include/swift/AST/DeclContext.h

@@ -610,10 +610,15 @@ class IterableDeclContext {
   mutable llvm::PointerIntPair<Decl *, 2, IterableDeclContextKind> 
     LastDeclAndKind;
 
-  // The DeclID this IDC was deserialized from, if any. Used for named lazy
-  // member loading, as a key when doing lookup in this IDC.
+  /// The DeclID this IDC was deserialized from, if any. Used for named lazy
+  /// member loading, as a key when doing lookup in this IDC.
   serialization::DeclID SerialID;
 
+  /// Lazy member loading has a variety of feedback loops that need to
+  /// switch to pseudo-empty-member behaviour to avoid infinite recursion;
+  /// we use this flag to control them.
+  bool lazyMemberLoadingInProgress = false;
+
   template<class A, class B, class C>
   friend struct ::llvm::cast_convert_val;
 
@@ -643,6 +648,14 @@ class IterableDeclContext {
     return FirstDeclAndLazyMembers.getInt();
   }
 
+  bool isLoadingLazyMembers() {
+    return lazyMemberLoadingInProgress;
+  }
+
+  void setLoadingLazyMembers(bool inProgress) {
+    lazyMemberLoadingInProgress = inProgress;
+  }
+
   /// Setup the loader for lazily-loaded members.
   void setMemberLoader(LazyMemberLoader *loader, uint64_t contextData);
 

lib/AST/NameLookup.cpp

@@ -1024,15 +1024,6 @@ class swift::MemberLookupTable {
     return Lookup.find(name);
   }
 
-  /// \brief Add an empty entry to the lookup map for a given name if it does
-  /// not yet have one.
-  void addEmptyEntry(DeclName name) {
-    (void)Lookup[name];
-    if (!name.isSimpleName()) {
-      (void)Lookup[name.getBaseName()];
-    }
-  }
-
   // \brief Mark all Decls in this table as not-resident in a table, drop
   // references to them. Should only be called when this was not fully-populated
   // from an IterableDeclContext.
@@ -1289,13 +1280,14 @@ populateLookupTableEntryFromLazyIDCLoader(ASTContext &ctx,
                                           MemberLookupTable &LookupTable,
                                           DeclName name,
                                           IterableDeclContext *IDC) {
+  if (IDC->isLoadingLazyMembers()) {
+    return false;
+  }
+  IDC->setLoadingLazyMembers(true);
   auto ci = ctx.getOrCreateLazyIterableContextData(IDC,
                                                    /*lazyLoader=*/nullptr);
-  // Populate LookupTable with an empty vector before we call into our loader,
-  // so that any reentry of this routine will find the set-so-far, and not
-  // fall into infinite recursion.
-  LookupTable.addEmptyEntry(name);
   if (auto res = ci->loader->loadNamedMembers(IDC, name, ci->memberData)) {
+    IDC->setLoadingLazyMembers(false);
     if (auto s = ctx.Stats) {
       ++s->getFrontendCounters().NamedLazyMemberLoadSuccessCount;
     }
@@ -1304,6 +1296,7 @@ populateLookupTableEntryFromLazyIDCLoader(ASTContext &ctx,
     }
     return false;
   } else {
+    IDC->setLoadingLazyMembers(false);
     if (auto s = ctx.Stats) {
       ++s->getFrontendCounters().NamedLazyMemberLoadFailureCount;
     }