[clang] AST: fix getAs canonicalization of leaf types

Before this patch, the application of getAs and castAs on a leaf type
would always produce a canonical type, which is undesirable because
some of these types can be sugared.

The user expectation is that getAs only removes top level sugar nodes, leaving
all the type sugar on the returned node, but it had an optimization intended
for type nodes with no sugar: for these, we can skip the expensive traversal
of the top level sugar with a simple canonicalization followed by dyn_cast.

The problem is that the concept of leaf type does not map well to what is
correct to apply this optimization to.

This patch replaces the concept of leaf types with 'always canonical' types,
and only applies the canonicalization strategy on them.

In order to avoid the performance regression this would cause, as most current
users do not care about type sugar, this patch also replaces all of these uses
with alternative cast functions which operate through canonicalization.

* Introduces castAs variants to complement the getAsTagDecl and derived
  variants.
* Introduces getAsEnumDecl and castAsEnumDecl, complementing the current set, so
  that all TagDecls are covered.
* Introduces getAsCanonical and castAsCanonical, for faster casting when only
  the canonical type is desired.

The getAsTagDecl and related functions are not provided inline, because
of the circular dependencies that would involve. So this patch causes a
small overall performance regression:
<img width="1461" height="18" alt="image" src="https://github.com/user-attachments/assets/061dfb14-9506-4623-91ec-0f02f585d1dd" />

This will be fixed in a later patch, bringing the whole thing back to a positive
performance improvement overall:
<img width="1462" height="18" alt="image" src="https://github.com/user-attachments/assets/c237e68f-f696-44f4-acc6-a7c7ba5b0976" />

Author: mizvekov

clang-tools-extra/clang-doc/Serialize.cpp

@@ -901,9 +901,8 @@ parseBases(RecordInfo &I, const CXXRecordDecl *D, bool IsFileInRootDir,
   if (!D->isThisDeclarationADefinition())
     return;
   for (const CXXBaseSpecifier &B : D->bases()) {
-    if (const RecordType *Ty = B.getType()->getAs<RecordType>()) {
-      if (const CXXRecordDecl *Base = cast_or_null<CXXRecordDecl>(
-              Ty->getOriginalDecl()->getDefinition())) {
+    if (const auto *Base = B.getType()->getAsCXXRecordDecl()) {
+      if (Base->isCompleteDefinition()) {
         // Initialized without USR and name, this will be set in the following
         // if-else stmt.
         BaseRecordInfo BI(

clang-tools-extra/clang-tidy/bugprone/EasilySwappableParametersCheck.cpp

@@ -997,7 +997,7 @@ approximateStandardConversionSequence(const TheCheck &Check, QualType From,
     WorkType = QualType{ToBuiltin, FastQualifiersToApply};
   }
 
-  const auto *FromEnum = WorkType->getAs<EnumType>();
+  const auto *FromEnum = WorkType->getAsCanonical<EnumType>();
   const auto *ToEnum = To->getAs<EnumType>();
   if (FromEnum && ToNumeric && FromEnum->isUnscopedEnumerationType()) {
     // Unscoped enumerations (or enumerations in C) convert to numerics.

clang-tools-extra/clang-tidy/bugprone/TaggedUnionMemberCountCheck.cpp

@@ -169,15 +169,8 @@ void TaggedUnionMemberCountCheck::check(
   if (!Root || !UnionField || !TagField)
     return;
 
-  const auto *UnionDef =
-      UnionField->getType().getCanonicalType().getTypePtr()->getAsRecordDecl();
-  const auto *EnumDef = llvm::dyn_cast<EnumDecl>(
-      TagField->getType().getCanonicalType().getTypePtr()->getAsTagDecl());
-
-  assert(UnionDef && "UnionDef is missing!");
-  assert(EnumDef && "EnumDef is missing!");
-  if (!UnionDef || !EnumDef)
-    return;
+  const auto *UnionDef = UnionField->getType()->castAsRecordDecl();
+  const auto *EnumDef = TagField->getType()->castAsEnumDecl();
 
   const std::size_t UnionMemberCount = llvm::range_size(UnionDef->fields());
   auto [TagCount, CountingEnumConstantDecl] = getNumberOfEnumValues(EnumDef);

clang-tools-extra/clang-tidy/cert/DefaultOperatorNewAlignmentCheck.cpp

@@ -31,7 +31,7 @@ void DefaultOperatorNewAlignmentCheck::check(
     return;
   const TagDecl *D = T->getAsTagDecl();
   // Alignment can not be obtained for undefined type.
-  if (!D || !D->getDefinition() || !D->isCompleteDefinition())
+  if (!D || !D->isCompleteDefinition())
     return;
 
   ASTContext &Context = D->getASTContext();

clang-tools-extra/clang-tidy/cppcoreguidelines/MissingStdForwardCheck.cpp

@@ -45,7 +45,7 @@ AST_MATCHER(ParmVarDecl, isTemplateTypeParameter) {
 
   QualType ParamType =
       Node.getType().getNonPackExpansionType()->getPointeeType();
-  const auto *TemplateType = ParamType->getAs<TemplateTypeParmType>();
+  const auto *TemplateType = ParamType->getAsCanonical<TemplateTypeParmType>();
   if (!TemplateType)
     return false;
 

clang-tools-extra/clang-tidy/cppcoreguidelines/ProTypeMemberInitCheck.cpp

@@ -189,7 +189,7 @@ struct InitializerInsertion {
 
 // Convenience utility to get a RecordDecl from a QualType.
 const RecordDecl *getCanonicalRecordDecl(const QualType &Type) {
-  if (const auto *RT = Type.getCanonicalType()->getAs<RecordType>())
+  if (const auto *RT = Type->getAsCanonical<RecordType>())
     return RT->getOriginalDecl();
   return nullptr;
 }

clang-tools-extra/clang-tidy/cppcoreguidelines/SlicingCheck.cpp

@@ -90,9 +90,8 @@ void SlicingCheck::diagnoseSlicedOverriddenMethods(
   }
   // Recursively process bases.
   for (const auto &Base : DerivedDecl.bases()) {
-    if (const auto *BaseRecordType = Base.getType()->getAs<RecordType>()) {
-      if (const auto *BaseRecord = cast_or_null<CXXRecordDecl>(
-              BaseRecordType->getOriginalDecl()->getDefinition()))
+    if (const auto *BaseRecord = Base.getType()->getAsCXXRecordDecl()) {
+      if (BaseRecord->isCompleteDefinition())
         diagnoseSlicedOverriddenMethods(Call, *BaseRecord, BaseDecl);
     }
   }

clang-tools-extra/clang-tidy/fuchsia/MultipleInheritanceCheck.cpp

@@ -71,13 +71,10 @@ bool MultipleInheritanceCheck::isInterface(const CXXRecordDecl *Node) {
   for (const auto &I : Node->bases()) {
     if (I.isVirtual())
       continue;
-    const auto *Ty = I.getType()->getAs<RecordType>();
-    if (!Ty)
+    const auto *Base = I.getType()->getAsCXXRecordDecl();
+    if (!Base)
       continue;
-    const RecordDecl *D = Ty->getOriginalDecl()->getDefinition();
-    if (!D)
-      continue;
-    const auto *Base = cast<CXXRecordDecl>(D);
+    assert(Base->isCompleteDefinition());
     if (!isInterface(Base)) {
       addNodeToInterfaceMap(Node, false);
       return false;
@@ -103,23 +100,21 @@ void MultipleInheritanceCheck::check(const MatchFinder::MatchResult &Result) {
     for (const auto &I : D->bases()) {
       if (I.isVirtual())
         continue;
-      const auto *Ty = I.getType()->getAs<RecordType>();
-      if (!Ty)
+      const auto *Base = I.getType()->getAsCXXRecordDecl();
+      if (!Base)
         continue;
-      const auto *Base =
-          cast<CXXRecordDecl>(Ty->getOriginalDecl()->getDefinition());
+      assert(Base->isCompleteDefinition());
       if (!isInterface(Base))
         NumConcrete++;
     }
 
     // Check virtual bases to see if there is more than one concrete
     // non-virtual base.
     for (const auto &V : D->vbases()) {
-      const auto *Ty = V.getType()->getAs<RecordType>();
-      if (!Ty)
+      const auto *Base = V.getType()->getAsCXXRecordDecl();
+      if (!Base)
         continue;
-      const auto *Base =
-          cast<CXXRecordDecl>(Ty->getOriginalDecl()->getDefinition());
+      assert(Base->isCompleteDefinition());
       if (!isInterface(Base))
         NumConcrete++;
     }

clang-tools-extra/clang-tidy/misc/UnusedUsingDeclsCheck.cpp

@@ -131,7 +131,7 @@ void UnusedUsingDeclsCheck::check(const MatchFinder::MatchResult &Result) {
       return;
     }
     if (const auto *ECD = dyn_cast<EnumConstantDecl>(Used)) {
-      if (const auto *ET = ECD->getType()->getAs<EnumType>())
+      if (const auto *ET = ECD->getType()->getAsCanonical<EnumType>())
         removeFromFoundDecls(ET->getOriginalDecl());
     }
   };

clang-tools-extra/clang-tidy/modernize/UseScopedLockCheck.cpp

@@ -28,7 +28,7 @@ static bool isLockGuardDecl(const NamedDecl *Decl) {
 }
 
 static bool isLockGuard(const QualType &Type) {
-  if (const auto *Record = Type->getAs<RecordType>())
+  if (const auto *Record = Type->getAsCanonical<RecordType>())
     if (const RecordDecl *Decl = Record->getOriginalDecl())
       return isLockGuardDecl(Decl);