[clang] fix broken canonicalization of DeducedTemplateSpecializationType (#95202)

This reverts the functional elements of commit
3e78fa8 and redoes it, by fixing the
true root cause of #61317.

A TemplateName can be non-canonical; profiling it based on the canonical
name would result in inconsistent preservation of as-written information
in the AST.

The true problem in #61317 is that we would not consider the methods
with requirements expression which contain DTSTs as the same in relation
to merging of declarations when importing modules.

The expressions would never match because they contained DTSTs pointing
to different redeclarations of the same class template, but since
canonicalization for them was broken, their canonical types would not
match either.

---

No changelog entry because #61317 was already claimed as fixed in
previous release.

Author: mizvekov

clang/include/clang/AST/ASTContext.h

@@ -1771,6 +1771,13 @@ class ASTContext : public RefCountedBase<ASTContext> {
                                                 QualType DeducedType,
                                                 bool IsDependent) const;
 
+private:
+  QualType getDeducedTemplateSpecializationTypeInternal(TemplateName Template,
+                                                        QualType DeducedType,
+                                                        bool IsDependent,
+                                                        QualType Canon) const;
+
+public:
   /// Return the unique reference to the type for the specified TagDecl
   /// (struct/union/class/enum) decl.
   QualType getTagDeclType(const TagDecl *Decl) const;

clang/include/clang/AST/TemplateName.h

@@ -346,7 +346,9 @@ class TemplateName {
   /// error.
   void dump() const;
 
-  void Profile(llvm::FoldingSetNodeID &ID);
+  void Profile(llvm::FoldingSetNodeID &ID) {
+    ID.AddPointer(Storage.getOpaqueValue());
+  }
 
   /// Retrieve the template name as a void pointer.
   void *getAsVoidPointer() const { return Storage.getOpaqueValue(); }

clang/include/clang/AST/Type.h

@@ -6050,30 +6050,27 @@ class DeducedTemplateSpecializationType : public DeducedType,
 
   DeducedTemplateSpecializationType(TemplateName Template,
                                     QualType DeducedAsType,
-                                    bool IsDeducedAsDependent)
+                                    bool IsDeducedAsDependent, QualType Canon)
       : DeducedType(DeducedTemplateSpecialization, DeducedAsType,
                     toTypeDependence(Template.getDependence()) |
                         (IsDeducedAsDependent
                              ? TypeDependence::DependentInstantiation
                              : TypeDependence::None),
-                    DeducedAsType.isNull() ? QualType(this, 0)
-                                           : DeducedAsType.getCanonicalType()),
+                    Canon),
         Template(Template) {}
 
 public:
   /// Retrieve the name of the template that we are deducing.
   TemplateName getTemplateName() const { return Template;}
 
-  void Profile(llvm::FoldingSetNodeID &ID) {
+  void Profile(llvm::FoldingSetNodeID &ID) const {
     Profile(ID, getTemplateName(), getDeducedType(), isDependentType());
   }
 
   static void Profile(llvm::FoldingSetNodeID &ID, TemplateName Template,
                       QualType Deduced, bool IsDependent) {
     Template.Profile(ID);
-    QualType CanonicalType =
-        Deduced.isNull() ? Deduced : Deduced.getCanonicalType();
-    ID.AddPointer(CanonicalType.getAsOpaquePtr());
+    Deduced.Profile(ID);
     ID.AddBoolean(IsDependent || Template.isDependent());
   }
 

clang/lib/AST/ASTContext.cpp

@@ -5926,11 +5926,9 @@ QualType ASTContext::getUnconstrainedType(QualType T) const {
   return T;
 }
 
-/// Return the uniqued reference to the deduced template specialization type
-/// which has been deduced to the given type, or to the canonical undeduced
-/// such type, or the canonical deduced-but-dependent such type.
-QualType ASTContext::getDeducedTemplateSpecializationType(
-    TemplateName Template, QualType DeducedType, bool IsDependent) const {
+QualType ASTContext::getDeducedTemplateSpecializationTypeInternal(
+    TemplateName Template, QualType DeducedType, bool IsDependent,
+    QualType Canon) const {
   // Look in the folding set for an existing type.
   void *InsertPos = nullptr;
   llvm::FoldingSetNodeID ID;
@@ -5941,7 +5939,8 @@ QualType ASTContext::getDeducedTemplateSpecializationType(
     return QualType(DTST, 0);
 
   auto *DTST = new (*this, alignof(DeducedTemplateSpecializationType))
-      DeducedTemplateSpecializationType(Template, DeducedType, IsDependent);
+      DeducedTemplateSpecializationType(Template, DeducedType, IsDependent,
+                                        Canon);
   llvm::FoldingSetNodeID TempID;
   DTST->Profile(TempID);
   assert(ID == TempID && "ID does not match");
@@ -5950,6 +5949,20 @@ QualType ASTContext::getDeducedTemplateSpecializationType(
   return QualType(DTST, 0);
 }
 
+/// Return the uniqued reference to the deduced template specialization type
+/// which has been deduced to the given type, or to the canonical undeduced
+/// such type, or the canonical deduced-but-dependent such type.
+QualType ASTContext::getDeducedTemplateSpecializationType(
+    TemplateName Template, QualType DeducedType, bool IsDependent) const {
+  QualType Canon = DeducedType.isNull()
+                       ? getDeducedTemplateSpecializationTypeInternal(
+                             getCanonicalTemplateName(Template), QualType(),
+                             IsDependent, QualType())
+                       : DeducedType.getCanonicalType();
+  return getDeducedTemplateSpecializationTypeInternal(Template, DeducedType,
+                                                      IsDependent, Canon);
+}
+
 /// getAtomicType - Return the uniqued reference to the atomic type for
 /// the given value type.
 QualType ASTContext::getAtomicType(QualType T) const {

clang/lib/AST/TemplateName.cpp

@@ -264,15 +264,6 @@ bool TemplateName::containsUnexpandedParameterPack() const {
   return getDependence() & TemplateNameDependence::UnexpandedPack;
 }
 
-void TemplateName::Profile(llvm::FoldingSetNodeID &ID) {
-  if (const auto* USD = getAsUsingShadowDecl())
-    ID.AddPointer(USD->getCanonicalDecl());
-  else if (const auto *TD = getAsTemplateDecl())
-    ID.AddPointer(TD->getCanonicalDecl());
-  else
-    ID.AddPointer(Storage.getOpaqueValue());
-}
-
 void TemplateName::print(raw_ostream &OS, const PrintingPolicy &Policy,
                          Qualified Qual) const {
   auto handleAnonymousTTP = [](TemplateDecl *TD, raw_ostream &OS) {

clang/unittests/AST/CMakeLists.txt

@@ -31,6 +31,7 @@ add_clang_unittest(ASTTests
   EvaluateAsRValueTest.cpp
   ExternalASTSourceTest.cpp
   NamedDeclPrinterTest.cpp
+  ProfilingTest.cpp
   RandstructTest.cpp
   RecursiveASTVisitorTest.cpp
   SizelessTypesTest.cpp

clang/unittests/AST/ProfilingTest.cpp

@@ -0,0 +1,73 @@
+//===- unittests/AST/ProfilingTest.cpp --- Tests for Profiling ------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "clang/AST/ASTContext.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
+#include "clang/Tooling/Tooling.h"
+#include "gtest/gtest.h"
+#include <utility>
+
+namespace clang {
+namespace {
+using namespace ast_matchers;
+
+static auto getClassTemplateRedecls() {
+  std::string Code = R"cpp(
+    template <class> struct A;
+    template <class> struct A;
+    template <class> struct A;
+  )cpp";
+  auto AST = tooling::buildASTFromCode(Code);
+  ASTContext &Ctx = AST->getASTContext();
+
+  auto MatchResults = match(classTemplateDecl().bind("id"), Ctx);
+  SmallVector<ClassTemplateDecl *, 3> Res;
+  for (BoundNodes &N : MatchResults) {
+    if (auto *CTD = const_cast<ClassTemplateDecl *>(
+            N.getNodeAs<ClassTemplateDecl>("id")))
+      Res.push_back(CTD);
+  }
+  assert(Res.size() == 3);
+  for (auto &&I : Res)
+    assert(I->getCanonicalDecl() == Res[0]);
+  return std::make_tuple(std::move(AST), Res[1], Res[2]);
+}
+
+template <class T> static void testTypeNode(const T *T1, const T *T2) {
+  {
+    llvm::FoldingSetNodeID ID1, ID2;
+    T1->Profile(ID1);
+    T2->Profile(ID2);
+    ASSERT_NE(ID1, ID2);
+  }
+  auto *CT1 = cast<T>(T1->getCanonicalTypeInternal());
+  auto *CT2 = cast<T>(T2->getCanonicalTypeInternal());
+  {
+    llvm::FoldingSetNodeID ID1, ID2;
+    CT1->Profile(ID1);
+    CT2->Profile(ID2);
+    ASSERT_EQ(ID1, ID2);
+  }
+}
+
+TEST(Profiling, DeducedTemplateSpecializationType_Name) {
+  auto [AST, CTD1, CTD2] = getClassTemplateRedecls();
+  ASTContext &Ctx = AST->getASTContext();
+
+  auto *T1 = cast<DeducedTemplateSpecializationType>(
+      Ctx.getDeducedTemplateSpecializationType(TemplateName(CTD1), QualType(),
+                                               false));
+  auto *T2 = cast<DeducedTemplateSpecializationType>(
+      Ctx.getDeducedTemplateSpecializationType(TemplateName(CTD2), QualType(),
+                                               false));
+  testTypeNode(T1, T2);
+}
+
+} // namespace
+} // namespace clang