[clang] Fix non-deterministic infinite recursion... (#118288)

...in `ASTContext::getAutoTypeInternal`

Given

```cpp
template < typename >
concept C1 = true;

template < typename , auto >
concept C2 = true;

template < C1 auto V, C2< V > auto>
struct S;
```

Both `C1 auto V` and `C2<V> auto` end on the set `AutoType`, the former
being a template parameter for the latter.

Since the hashing is not deterministic (i.e., pointers are hashed),
every now and then, both will end on the same bucket. Given that
`FoldingSet` recomputes the `FoldingSetID` for each node in the target
bucket on lookup, this triggers an infinite recursion:

1. Look for `X` in `AutoTypes`
2. Let's assume it would be in bucket N, so it iterates over nodes in
that bucket. Let's assume the first is `C2<V> auto`.
3. Computes the `FoldingSetID` for this one, which requires the profile
of its template parameters, so they are visited.
4. In some frames below, we end on the same `FoldingSet`, and, by
chance, `C1 auto V` would be in bucket N too.
5. But the first node in the bucket is `C2<V> auto` for which we need to
profile `C1 auto V`
6. ... stack overflow!

No step individually does anything wrong, but in general, `FoldingSet`
seems not to be re-entrant, and this fact is hidden behind many nested
calls.

With this change, we store the `AutoType`s inside a `DenseMap` instead.
The `FoldingSetID` is computed once only and then kept as the map's key,
avoiding the need to do recursive lookups.

We also now make sure the key for the inserted `AutoType` is the same as
the key used for lookup. Before, this was not the case, and it caused
also non-deterministic parsing errors.

Fixes https://github.com/llvm/llvm-project/issues/110231

clang/include/clang/AST/ASTContext.h

@@ -245,7 +245,11 @@ class ASTContext : public RefCountedBase<ASTContext> {
   mutable llvm::FoldingSet<ObjCObjectPointerType> ObjCObjectPointerTypes;
   mutable llvm::FoldingSet<DependentUnaryTransformType>
     DependentUnaryTransformTypes;
-  mutable llvm::ContextualFoldingSet<AutoType, ASTContext&> AutoTypes;
+  // An AutoType can have a dependency on another AutoType via its template
+  // arguments. Since both dependent and dependency are on the same set,
+  // we can end up in an infinite recursion when looking for a node if we used
+  // a `FoldingSet`, since both could end up in the same bucket.
+  mutable llvm::DenseMap<llvm::FoldingSetNodeID, AutoType *> AutoTypes;
   mutable llvm::FoldingSet<DeducedTemplateSpecializationType>
     DeducedTemplateSpecializationTypes;
   mutable llvm::FoldingSet<AtomicType> AtomicTypes;

clang/include/clang/AST/Type.h

@@ -6553,7 +6553,7 @@ public:
 
 /// Represents a C++11 auto or C++14 decltype(auto) type, possibly constrained
 /// by a type-constraint.
-class AutoType : public DeducedType, public llvm::FoldingSetNode {
+class AutoType : public DeducedType {
   friend class ASTContext; // ASTContext creates these
 
   ConceptDecl *TypeConstraintConcept;

clang/lib/AST/ASTContext.cpp

@@ -112,6 +112,27 @@ enum FloatingRank {
   Ibm128Rank
 };
 
+template <> struct llvm::DenseMapInfo<llvm::FoldingSetNodeID> {
+  static FoldingSetNodeID getEmptyKey() { return FoldingSetNodeID{}; }
+
+  static FoldingSetNodeID getTombstoneKey() {
+    FoldingSetNodeID id;
+    for (size_t i = 0; i < sizeof(id) / sizeof(unsigned); ++i) {
+      id.AddInteger(std::numeric_limits<unsigned>::max());
+    }
+    return id;
+  }
+
+  static unsigned getHashValue(const FoldingSetNodeID &Val) {
+    return Val.ComputeHash();
+  }
+
+  static bool isEqual(const FoldingSetNodeID &LHS,
+                      const FoldingSetNodeID &RHS) {
+    return LHS == RHS;
+  }
+};
+
 /// \returns The locations that are relevant when searching for Doc comments
 /// related to \p D.
 static SmallVector<SourceLocation, 2>
@@ -899,7 +920,7 @@ ASTContext::ASTContext(LangOptions &LOpts, SourceManager &SM,
       FunctionProtoTypes(this_(), FunctionProtoTypesLog2InitSize),
       DependentTypeOfExprTypes(this_()), DependentDecltypeTypes(this_()),
       TemplateSpecializationTypes(this_()),
-      DependentTemplateSpecializationTypes(this_()), AutoTypes(this_()),
+      DependentTemplateSpecializationTypes(this_()),
       DependentBitIntTypes(this_()), SubstTemplateTemplateParmPacks(this_()),
       DeducedTemplates(this_()), ArrayParameterTypes(this_()),
       CanonTemplateTemplateParms(this_()), SourceMgr(SM), LangOpts(LOpts),
@@ -6294,12 +6315,14 @@ QualType ASTContext::getAutoTypeInternal(
     return getAutoDeductType();
 
   // Look in the folding set for an existing type.
-  void *InsertPos = nullptr;
   llvm::FoldingSetNodeID ID;
-  AutoType::Profile(ID, *this, DeducedType, Keyword, IsDependent,
-                    TypeConstraintConcept, TypeConstraintArgs);
-  if (AutoType *AT = AutoTypes.FindNodeOrInsertPos(ID, InsertPos))
-    return QualType(AT, 0);
+  bool IsDeducedDependent =
+      !DeducedType.isNull() && DeducedType->isDependentType();
+  AutoType::Profile(ID, *this, DeducedType, Keyword,
+                    IsDependent || IsDeducedDependent, TypeConstraintConcept,
+                    TypeConstraintArgs);
+  if (auto const AT_iter = AutoTypes.find(ID); AT_iter != AutoTypes.end())
+    return QualType(AT_iter->getSecond(), 0);
 
   QualType Canon;
   if (!IsCanon) {
@@ -6314,10 +6337,6 @@ QualType ASTContext::getAutoTypeInternal(
         Canon =
             getAutoTypeInternal(QualType(), Keyword, IsDependent, IsPack,
                                 CanonicalConcept, CanonicalConceptArgs, true);
-        // Find the insert position again.
-        [[maybe_unused]] auto *Nothing =
-            AutoTypes.FindNodeOrInsertPos(ID, InsertPos);
-        assert(!Nothing && "canonical type broken");
       }
     }
   }
@@ -6331,8 +6350,13 @@ QualType ASTContext::getAutoTypeInternal(
                    : TypeDependence::None) |
           (IsPack ? TypeDependence::UnexpandedPack : TypeDependence::None),
       Canon, TypeConstraintConcept, TypeConstraintArgs);
+#ifndef NDEBUG
+  llvm::FoldingSetNodeID InsertedID;
+  AT->Profile(InsertedID, *this);
+  assert(InsertedID == ID && "ID does not match");
+#endif
   Types.push_back(AT);
-  AutoTypes.InsertNode(AT, InsertPos);
+  AutoTypes.try_emplace(ID, AT);
   return QualType(AT, 0);
 }
 

clang/test/Parser/gh110231.cpp

@@ -0,0 +1,12 @@
+// RUN: seq 100 | xargs -Ifoo %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+// expected-no-diagnostics
+// This is a regression test for a non-deterministic stack-overflow.
+
+template < typename >
+concept C1 = true;
+
+template < typename , auto >
+concept C2 = true;
+
+template < C1 auto V, C2< V > auto>
+struct S;