[clang] Track source deduction guide for alias template deduction guides (#123875)

For deduction guides generated from alias template CTAD, store the
deduction guide they were originated from. The source kind is also
maintained for future expansion in CTAD from inherited constructors.

This tracking is required to determine whether an alias template already
has a deduction guide corresponding to some deduction guide on the
original template, in order to support deduction guides for the alias
from deduction guides declared after the initial usage.

Author: antangelo

clang/include/clang/AST/DeclCXX.h

@@ -1967,24 +1967,42 @@ class ExplicitSpecifier {
 class CXXDeductionGuideDecl : public FunctionDecl {
   void anchor() override;
 
+public:
+  // Represents the relationship between this deduction guide and the
+  // deduction guide that it was generated from (or lack thereof).
+  // See the SourceDeductionGuide member for more details.
+  enum class SourceDeductionGuideKind : uint8_t {
+    None,
+    Alias,
+  };
+
 private:
   CXXDeductionGuideDecl(ASTContext &C, DeclContext *DC, SourceLocation StartLoc,
                         ExplicitSpecifier ES,
                         const DeclarationNameInfo &NameInfo, QualType T,
                         TypeSourceInfo *TInfo, SourceLocation EndLocation,
                         CXXConstructorDecl *Ctor, DeductionCandidate Kind,
-                        Expr *TrailingRequiresClause)
+                        Expr *TrailingRequiresClause,
+                        const CXXDeductionGuideDecl *GeneratedFrom,
+                        SourceDeductionGuideKind SourceKind)
       : FunctionDecl(CXXDeductionGuide, C, DC, StartLoc, NameInfo, T, TInfo,
                      SC_None, false, false, ConstexprSpecKind::Unspecified,
                      TrailingRequiresClause),
-        Ctor(Ctor), ExplicitSpec(ES) {
+        Ctor(Ctor), ExplicitSpec(ES),
+        SourceDeductionGuide(GeneratedFrom, SourceKind) {
     if (EndLocation.isValid())
       setRangeEnd(EndLocation);
     setDeductionCandidateKind(Kind);
   }
 
   CXXConstructorDecl *Ctor;
   ExplicitSpecifier ExplicitSpec;
+  // The deduction guide, if any, that this deduction guide was generated from,
+  // in the case of alias template deduction. The SourceDeductionGuideKind
+  // member indicates which of these sources applies, or is None otherwise.
+  llvm::PointerIntPair<const CXXDeductionGuideDecl *, 2,
+                       SourceDeductionGuideKind>
+      SourceDeductionGuide;
   void setExplicitSpecifier(ExplicitSpecifier ES) { ExplicitSpec = ES; }
 
 public:
@@ -1997,7 +2015,9 @@ class CXXDeductionGuideDecl : public FunctionDecl {
          TypeSourceInfo *TInfo, SourceLocation EndLocation,
          CXXConstructorDecl *Ctor = nullptr,
          DeductionCandidate Kind = DeductionCandidate::Normal,
-         Expr *TrailingRequiresClause = nullptr);
+         Expr *TrailingRequiresClause = nullptr,
+         const CXXDeductionGuideDecl *SourceDG = nullptr,
+         SourceDeductionGuideKind SK = SourceDeductionGuideKind::None);
 
   static CXXDeductionGuideDecl *CreateDeserialized(ASTContext &C,
                                                    GlobalDeclID ID);
@@ -2017,6 +2037,25 @@ class CXXDeductionGuideDecl : public FunctionDecl {
   /// this is an implicit deduction guide.
   CXXConstructorDecl *getCorrespondingConstructor() const { return Ctor; }
 
+  /// Get the deduction guide from which this deduction guide was generated,
+  /// if it was generated as part of alias template deduction or from an
+  /// inherited constructor.
+  const CXXDeductionGuideDecl *getSourceDeductionGuide() const {
+    return SourceDeductionGuide.getPointer();
+  }
+
+  void setSourceDeductionGuide(CXXDeductionGuideDecl *DG) {
+    SourceDeductionGuide.setPointer(DG);
+  }
+
+  SourceDeductionGuideKind getSourceDeductionGuideKind() const {
+    return SourceDeductionGuide.getInt();
+  }
+
+  void setSourceDeductionGuideKind(SourceDeductionGuideKind SK) {
+    SourceDeductionGuide.setInt(SK);
+  }
+
   void setDeductionCandidateKind(DeductionCandidate K) {
     FunctionDeclBits.DeductionCandidateKind = static_cast<unsigned char>(K);
   }

clang/lib/AST/ASTImporter.cpp

@@ -3997,14 +3997,16 @@ ExpectedDecl ASTNodeImporter::VisitFunctionDecl(FunctionDecl *D) {
         importExplicitSpecifier(Err, Guide->getExplicitSpecifier());
     CXXConstructorDecl *Ctor =
         importChecked(Err, Guide->getCorrespondingConstructor());
+    const CXXDeductionGuideDecl *SourceDG =
+        importChecked(Err, Guide->getSourceDeductionGuide());
     if (Err)
       return std::move(Err);
     if (GetImportedOrCreateDecl<CXXDeductionGuideDecl>(
             ToFunction, D, Importer.getToContext(), DC, ToInnerLocStart, ESpec,
-            NameInfo, T, TInfo, ToEndLoc, Ctor))
+            NameInfo, T, TInfo, ToEndLoc, Ctor,
+            Guide->getDeductionCandidateKind(), TrailingRequiresClause,
+            SourceDG, Guide->getSourceDeductionGuideKind()))
       return ToFunction;
-    cast<CXXDeductionGuideDecl>(ToFunction)
-        ->setDeductionCandidateKind(Guide->getDeductionCandidateKind());
   } else {
     if (GetImportedOrCreateDecl(
             ToFunction, D, Importer.getToContext(), DC, ToInnerLocStart,

clang/lib/AST/DeclCXX.cpp

@@ -2292,18 +2292,22 @@ CXXDeductionGuideDecl *CXXDeductionGuideDecl::Create(
     ASTContext &C, DeclContext *DC, SourceLocation StartLoc,
     ExplicitSpecifier ES, const DeclarationNameInfo &NameInfo, QualType T,
     TypeSourceInfo *TInfo, SourceLocation EndLocation, CXXConstructorDecl *Ctor,
-    DeductionCandidate Kind, Expr *TrailingRequiresClause) {
-  return new (C, DC)
-      CXXDeductionGuideDecl(C, DC, StartLoc, ES, NameInfo, T, TInfo,
-                            EndLocation, Ctor, Kind, TrailingRequiresClause);
+    DeductionCandidate Kind, Expr *TrailingRequiresClause,
+    const CXXDeductionGuideDecl *GeneratedFrom,
+    SourceDeductionGuideKind SourceKind) {
+  return new (C, DC) CXXDeductionGuideDecl(
+      C, DC, StartLoc, ES, NameInfo, T, TInfo, EndLocation, Ctor, Kind,
+      TrailingRequiresClause, GeneratedFrom, SourceKind);
 }
 
 CXXDeductionGuideDecl *
 CXXDeductionGuideDecl::CreateDeserialized(ASTContext &C, GlobalDeclID ID) {
   return new (C, ID) CXXDeductionGuideDecl(
-      C, nullptr, SourceLocation(), ExplicitSpecifier(), DeclarationNameInfo(),
-      QualType(), nullptr, SourceLocation(), nullptr,
-      DeductionCandidate::Normal, nullptr);
+      C, /*DC=*/nullptr, SourceLocation(), ExplicitSpecifier(),
+      DeclarationNameInfo(), QualType(), /*TInfo=*/nullptr, SourceLocation(),
+      /*Ctor=*/nullptr, DeductionCandidate::Normal,
+      /*TrailingRequiresClause=*/nullptr,
+      /*GeneratedFrom=*/nullptr, SourceDeductionGuideKind::None);
 }
 
 RequiresExprBodyDecl *RequiresExprBodyDecl::Create(

clang/lib/Sema/SemaTemplateDeductionGuide.cpp

@@ -1175,8 +1175,12 @@ BuildDeductionGuideForTypeAlias(Sema &SemaRef,
         GG->getTypeSourceInfo(), AliasTemplate->getBeginLoc(),
         AliasTemplate->getLocation(), AliasTemplate->getEndLoc(),
         F->isImplicit()));
-    cast<CXXDeductionGuideDecl>(Result->getTemplatedDecl())
-        ->setDeductionCandidateKind(GG->getDeductionCandidateKind());
+    auto *DGuide = cast<CXXDeductionGuideDecl>(Result->getTemplatedDecl());
+    DGuide->setDeductionCandidateKind(GG->getDeductionCandidateKind());
+    DGuide->setSourceDeductionGuide(
+        cast<CXXDeductionGuideDecl>(F->getTemplatedDecl()));
+    DGuide->setSourceDeductionGuideKind(
+        CXXDeductionGuideDecl::SourceDeductionGuideKind::Alias);
     return Result;
   }
   return nullptr;

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -2269,7 +2269,9 @@ Decl *TemplateDeclInstantiator::VisitFunctionDecl(
         SemaRef.Context, DC, D->getInnerLocStart(),
         InstantiatedExplicitSpecifier, NameInfo, T, TInfo,
         D->getSourceRange().getEnd(), DGuide->getCorrespondingConstructor(),
-        DGuide->getDeductionCandidateKind(), TrailingRequiresClause);
+        DGuide->getDeductionCandidateKind(), TrailingRequiresClause,
+        DGuide->getSourceDeductionGuide(),
+        DGuide->getSourceDeductionGuideKind());
     Function->setAccess(D->getAccess());
   } else {
     Function = FunctionDecl::Create(

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -2295,6 +2295,10 @@ void ASTDeclReader::VisitCXXDeductionGuideDecl(CXXDeductionGuideDecl *D) {
   VisitFunctionDecl(D);
   D->setDeductionCandidateKind(
       static_cast<DeductionCandidate>(Record.readInt()));
+  D->setSourceDeductionGuide(readDeclAs<CXXDeductionGuideDecl>());
+  D->setSourceDeductionGuideKind(
+      static_cast<CXXDeductionGuideDecl::SourceDeductionGuideKind>(
+          Record.readInt()));
 }
 
 void ASTDeclReader::VisitCXXMethodDecl(CXXMethodDecl *D) {

clang/lib/Serialization/ASTWriterDecl.cpp

@@ -846,6 +846,9 @@ void ASTDeclWriter::VisitCXXDeductionGuideDecl(CXXDeductionGuideDecl *D) {
   Record.AddDeclRef(D->Ctor);
   VisitFunctionDecl(D);
   Record.push_back(static_cast<unsigned char>(D->getDeductionCandidateKind()));
+  Record.AddDeclRef(D->getSourceDeductionGuide());
+  Record.push_back(
+      static_cast<unsigned char>(D->getSourceDeductionGuideKind()));
   Code = serialization::DECL_CXX_DEDUCTION_GUIDE;
 }
 

clang/unittests/AST/ASTImporterTest.cpp

@@ -8125,6 +8125,9 @@ TEST_P(ImportFunctions, CTADImplicit) {
   auto *ToD = Import(FromD, Lang_CXX17);
   ASSERT_TRUE(ToD);
   EXPECT_EQ(ToD->getDeductionCandidateKind(), DeductionCandidate::Copy);
+  EXPECT_EQ(ToD->getSourceDeductionGuide(), nullptr);
+  EXPECT_EQ(ToD->getSourceDeductionGuideKind(),
+            CXXDeductionGuideDecl::SourceDeductionGuideKind::None);
   // Check that the deduced class template is also imported.
   EXPECT_TRUE(findFromTU(FromD)->Importer->GetAlreadyImportedOrNull(
       FromD->getDeducedTemplate()));
@@ -8149,6 +8152,9 @@ TEST_P(ImportFunctions, CTADUserDefinedExplicit) {
   ASSERT_TRUE(ToD);
   EXPECT_FALSE(FromD->isImplicit());
   EXPECT_TRUE(ToD->isExplicit());
+  EXPECT_EQ(ToD->getSourceDeductionGuide(), nullptr);
+  EXPECT_EQ(ToD->getSourceDeductionGuideKind(),
+            CXXDeductionGuideDecl::SourceDeductionGuideKind::None);
 }
 
 TEST_P(ImportFunctions, CTADWithLocalTypedef) {
@@ -8167,6 +8173,26 @@ TEST_P(ImportFunctions, CTADWithLocalTypedef) {
   ASSERT_TRUE(ToD);
 }
 
+TEST_P(ImportFunctions, CTADAliasTemplate) {
+  Decl *TU = getTuDecl(
+      R"(
+      template <typename T> struct A {
+        A(T);
+      };
+      template<typename T>
+      using B = A<T>;
+      B b{(int)0};
+      )",
+      Lang_CXX20, "input.cc");
+  auto *FromD = FirstDeclMatcher<CXXDeductionGuideDecl>().match(
+      TU, cxxDeductionGuideDecl(hasParameter(0, hasType(asString("int")))));
+  auto *ToD = Import(FromD, Lang_CXX20);
+  ASSERT_TRUE(ToD);
+  EXPECT_TRUE(ToD->getSourceDeductionGuideKind() ==
+              CXXDeductionGuideDecl::SourceDeductionGuideKind::Alias);
+  EXPECT_TRUE(ToD->getSourceDeductionGuide());
+}
+
 TEST_P(ImportFunctions, ParmVarDeclDeclContext) {
   constexpr auto FromTUCode = R"(
       void f(int P);