[clang] Fix issue with FoldingSet and DependentTemplateSpecializationType

PR llvm#118288 fixed a re-entrancy issue with the usage of `FoldingSet`
and `AutoType`.  The following test case (reduced with `creduce` by
@Fznamznon):

```
template <typename C, typename S1, int rbits>
typename C::A Bar(const S1& x, const C& c = C()) {
    using T = typename C::A;
    T result;

    using PreC = typename C::template boop<T::p + rbits>;
    using ExactC = typename C::template bap<PreC::p + 2>;

    using D = typename ExactC::A;

    return result;
}
```

shows a similar non-deterministic recursion with the use of
`FoldingSet` but with `DependentTemplateSepcializationType`.
A nearly identical fix is needed here too.

Author: premanandrao

clang/include/clang/AST/ASTContext.h

@@ -238,9 +238,9 @@ class ASTContext : public RefCountedBase<ASTContext> {
   mutable llvm::FoldingSet<ElaboratedType> ElaboratedTypes{
       GeneralTypesLog2InitSize};
   mutable llvm::FoldingSet<DependentNameType> DependentNameTypes;
-  mutable llvm::ContextualFoldingSet<DependentTemplateSpecializationType,
-                                     ASTContext&>
-    DependentTemplateSpecializationTypes;
+  mutable llvm::DenseMap<llvm::FoldingSetNodeID,
+                         DependentTemplateSpecializationType *>
+      DependentTemplateSpecializationTypes;
   mutable llvm::FoldingSet<PackExpansionType> PackExpansionTypes;
   mutable llvm::FoldingSet<ObjCObjectTypeImpl> ObjCObjectTypes;
   mutable llvm::FoldingSet<ObjCObjectPointerType> ObjCObjectPointerTypes;

clang/include/clang/AST/Type.h

@@ -7276,8 +7276,7 @@ class DependentNameType : public TypeWithKeyword, public llvm::FoldingSetNode {
 /// Represents a template specialization type whose template cannot be
 /// resolved, e.g.
 ///   A<T>::template B<T>
-class DependentTemplateSpecializationType : public TypeWithKeyword,
-                                            public llvm::FoldingSetNode {
+class DependentTemplateSpecializationType : public TypeWithKeyword {
   friend class ASTContext; // ASTContext creates these
 
   DependentTemplateStorage Name;

clang/lib/AST/ASTContext.cpp

@@ -940,7 +940,6 @@ ASTContext::ASTContext(LangOptions &LOpts, SourceManager &SM,
       FunctionProtoTypes(this_(), FunctionProtoTypesLog2InitSize),
       DependentTypeOfExprTypes(this_()), DependentDecltypeTypes(this_()),
       DependentPackIndexingTypes(this_()), TemplateSpecializationTypes(this_()),
-      DependentTemplateSpecializationTypes(this_()),
       DependentBitIntTypes(this_()), SubstTemplateTemplateParmPacks(this_()),
       DeducedTemplates(this_()), ArrayParameterTypes(this_()),
       CanonTemplateTemplateParms(this_()), SourceMgr(SM), LangOpts(LOpts),
@@ -5979,10 +5978,9 @@ QualType ASTContext::getDependentTemplateSpecializationType(
   llvm::FoldingSetNodeID ID;
   DependentTemplateSpecializationType::Profile(ID, *this, Keyword, Name, Args);
 
-  void *InsertPos = nullptr;
-  if (auto *T = DependentTemplateSpecializationTypes.FindNodeOrInsertPos(
-          ID, InsertPos))
-    return QualType(T, 0);
+  if (auto const T_iter = DependentTemplateSpecializationTypes.find(ID);
+      T_iter != DependentTemplateSpecializationTypes.end())
+    return QualType(T_iter->getSecond(), 0);
 
   NestedNameSpecifier *NNS = Name.getQualifier();
 
@@ -6001,11 +5999,6 @@ QualType ASTContext::getDependentTemplateSpecializationType(
           CanonKeyword, {CanonNNS, Name.getName(), /*HasTemplateKeyword=*/true},
           CanonArgs,
           /*IsCanonical=*/true);
-      // Find the insert position again.
-      [[maybe_unused]] auto *Nothing =
-          DependentTemplateSpecializationTypes.FindNodeOrInsertPos(ID,
-                                                                   InsertPos);
-      assert(!Nothing && "canonical type broken");
     }
   } else {
     assert(Keyword == getCanonicalElaboratedTypeKeyword(Keyword));
@@ -6021,8 +6014,13 @@ QualType ASTContext::getDependentTemplateSpecializationType(
                        alignof(DependentTemplateSpecializationType));
   auto *T =
       new (Mem) DependentTemplateSpecializationType(Keyword, Name, Args, Canon);
+#ifndef NDEBUG
+  llvm::FoldingSetNodeID InsertedID;
+  T->Profile(InsertedID, *this);
+  assert(InsertedID == ID && "ID does not match");
+#endif
   Types.push_back(T);
-  DependentTemplateSpecializationTypes.InsertNode(T, InsertPos);
+  DependentTemplateSpecializationTypes.try_emplace(ID, T);
   return QualType(T, 0);
 }
 

clang/test/Parser/dep_template_spec_type.cpp

@@ -0,0 +1,16 @@
+// RUN: seq 100 | xargs -Ifoo %clang_cc1 -fsyntax-only -verify %s
+// expected-no-diagnostics
+// This is a regression test for a non-deterministic stack-overflow.
+
+template <typename C, typename S1, int rbits>
+typename C::A Bar(const S1& x, const C& c = C()) {
+    using T = typename C::A;
+    T result;
+
+    using PreC = typename C::template boop<T::p + rbits>;
+    using ExactC = typename C::template bap<PreC::p + 2>;
+
+    using D = typename ExactC::A;
+
+    return result;
+}