Sema: Teach TypeReprCycleCheckWalker to avoid more cycles

More declaration order issues uncovered by lazy type checking.

Fixes rdar://119499800.

lib/Sema/TypeCheckProtocolInference.cpp

@@ -165,27 +165,25 @@ namespace {
 /// Try to avoid situations where resolving the type of a witness calls back
 /// into associated type inference.
 struct TypeReprCycleCheckWalker : ASTWalker {
+  ASTContext &ctx;
   llvm::SmallDenseSet<Identifier, 2> circularNames;
   ValueDecl *witness;
   bool found;
 
   TypeReprCycleCheckWalker(
+      ASTContext &ctx,
       const llvm::SetVector<AssociatedTypeDecl *> &allUnresolved)
-    : witness(nullptr), found(false) {
+    : ctx(ctx), witness(nullptr), found(false) {
     for (auto *assocType : allUnresolved) {
       circularNames.insert(assocType->getName());
     }
   }
 
   PreWalkAction walkToTypeReprPre(TypeRepr *T) override {
-    // FIXME: We should still visit any generic arguments of this member type.
-    // However, we want to skip 'Foo.Element' because the 'Element' reference is
-    // not unqualified.
-    if (auto *memberTyR = dyn_cast<MemberTypeRepr>(T)) {
-      return Action::SkipChildren();
-    }
+    // FIXME: Visit generic arguments.
 
     if (auto *identTyR = dyn_cast<SimpleIdentTypeRepr>(T)) {
+      // If we're inferring `Foo`, don't look at a witness mentioning `Foo`.
       if (circularNames.count(identTyR->getNameRef().getBaseIdentifier()) > 0) {
         // If unqualified lookup can find a type with this name without looking
         // into protocol members, don't skip the witness, since this type might
@@ -194,7 +192,6 @@ struct TypeReprCycleCheckWalker : ASTWalker {
             identTyR->getNameRef(), witness->getDeclContext(),
             identTyR->getLoc(), UnqualifiedLookupOptions());
 
-        auto &ctx = witness->getASTContext();
         auto results =
             evaluateOrDefault(ctx.evaluator, UnqualifiedLookupRequest{desc}, {});
 
@@ -207,6 +204,34 @@ struct TypeReprCycleCheckWalker : ASTWalker {
       }
     }
 
+    if (auto *memberTyR = dyn_cast<MemberTypeRepr>(T)) {
+      // If we're looking at a member type`Foo.Bar`, check `Foo` recursively.
+      auto *baseTyR = memberTyR->getBaseComponent();
+      baseTyR->walk(*this);
+
+      // If we're inferring `Foo`, don't look at a witness mentioning `Self.Foo`.
+      if (auto *identTyR = dyn_cast<SimpleIdentTypeRepr>(baseTyR)) {
+        if (identTyR->getNameRef().getBaseIdentifier() == ctx.Id_Self) {
+          // But if qualified lookup can find a type with this name without
+          // looking into protocol members, don't skip the witness, since this
+          // type might be a candidate witness.
+          SmallVector<ValueDecl *, 2> results;
+          witness->getInnermostDeclContext()->lookupQualified(
+              witness->getDeclContext()->getSelfTypeInContext(),
+              identTyR->getNameRef(), SourceLoc(), NLOptions(), results);
+
+          // Ok, resolving this member type would trigger associated type
+          // inference recursively. We're going to skip this witness.
+          if (results.empty()) {
+            found = true;
+            return Action::Stop();
+          }
+        }
+      }
+
+      return Action::SkipChildren();
+    }
+
     return Action::Continue();
   }
 
@@ -296,7 +321,7 @@ AssociatedTypeInference::inferTypeWitnessesViaValueWitnesses(
     abort();
   }
 
-  TypeReprCycleCheckWalker cycleCheck(allUnresolved);
+  TypeReprCycleCheckWalker cycleCheck(dc->getASTContext(), allUnresolved);
 
   InferredAssociatedTypesByWitnesses result;
 

test/decl/protocol/req/assoc_type_inference_cycle.swift

@@ -95,3 +95,39 @@ public enum CaseWitness: CaseProtocol {
   case b(_: A)
   case c(_: A)
 }
+
+// rdar://119499800 #1
+public typealias A8 = Batch.Iterator
+
+public struct Batch: Collection {
+  public typealias Element = Int
+  public typealias Index = Array<Element>.Index
+
+  var elements: [Element]
+
+  init(_ elements: some Collection<Element>) {
+    self.elements = Array(elements)
+  }
+
+  public var startIndex: Index { return elements.startIndex }
+  public var endIndex: Index { return elements.endIndex }
+
+  public subscript(index: Index) -> Iterator.Element {
+    return elements[index]
+  }
+
+  public func index(after i: Index) -> Index {
+    return elements.index(after: i)
+  }
+}
+
+// rdar://119499800 #2
+public typealias A9 = LogTypes.RawValue
+
+public struct LogTypes: OptionSet {
+  public init(rawValue: Self.RawValue) {
+    self.rawValue = rawValue
+  }
+
+  public let rawValue: Int
+}