[clang] [clang] Implement P2582R1: CTAD from inherited constructors (PR #98788)

[via cfe-commits]

================
@@ -1216,10 +1308,225 @@ void DeclareImplicitDeductionGuidesForTypeAlias(
             ->getDeductionCandidateKind() == DeductionCandidate::Aggregate)
       continue;
 
-    BuildDeductionGuideForTypeAlias(SemaRef, AliasTemplate, F, Loc);
+    BuildDeductionGuideForTypeAlias(SemaRef, AliasTemplate, F, Loc,
+                                    DeducingTemplate, DerivedClassMapperType);
   }
 }
 
+void DeclareImplicitDeductionGuidesFromInheritedConstructors(
+    Sema &SemaRef, TemplateDecl *Template, ClassTemplateDecl *Pattern,
+    TypeSourceInfo *BaseTSI, unsigned BaseIdx) {
+  auto &Context = SemaRef.Context;
+  DeclContext *DC = Template->getDeclContext();
+  const auto *BaseTST = BaseTSI->getType()->getAs<TemplateSpecializationType>();
+  if (!BaseTST)
+    return;
+  SourceLocation BaseLoc = BaseTSI->getTypeLoc().getBeginLoc();
+
+  TemplateDecl *BaseTD = BaseTST->getTemplateName().getAsTemplateDecl();
+  if (!BaseTD)
+    return;
+
+  // Subsitute any parameters with default arguments not present in the base,
+  // since partial specializations cannot have default parameters
+  TemplateParameterList *TemplateTPL = Pattern->getTemplateParameters();
+  auto BaseDeducedTemplateParams =
+      TemplateParamsReferencedInTemplateArgumentList(
+          TemplateTPL, BaseTST->template_arguments());
+  SmallVector<NamedDecl *, 8> PartialSpecParams;
+  SmallVector<TemplateArgument, 8> SubstArgs;
+  PartialSpecParams.reserve(TemplateTPL->size());
+  SubstArgs.reserve(TemplateTPL->size());
+  LocalInstantiationScope Scope(SemaRef);
+  for (unsigned I = 0, N = TemplateTPL->size(); I < N; ++I) {
+    NamedDecl *Param = TemplateTPL->getParam(I);
+    if (!BaseDeducedTemplateParams.contains(I)) {
+      if (auto *TTP = dyn_cast<TemplateTypeParmDecl>(Param);
+          TTP && TTP->hasDefaultArgument()) {
+        SubstArgs.push_back(TTP->getDefaultArgument().getArgument());
+        continue;
+      }
+
+      if (auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(Param);
+          NTTP && NTTP->hasDefaultArgument()) {
+        SubstArgs.push_back(NTTP->getDefaultArgument().getArgument());
+        continue;
+      }
+
+      if (auto *TTP = dyn_cast<TemplateTemplateParmDecl>(Param);
+          TTP && TTP->hasDefaultArgument()) {
+        SubstArgs.push_back(TTP->getDefaultArgument().getArgument());
+        continue;
+      }
+
+      // We have a template parameter that is not present in the base
+      // and does not have a default argument. We create the deduction
+      // guide anyway to display a diagnostic.
+    }
+
+    MultiLevelTemplateArgumentList Args;
+    Args.setKind(TemplateSubstitutionKind::Rewrite);
+    Args.addOuterTemplateArguments(SubstArgs);
+    Args.addOuterRetainedLevels(Template->getTemplateDepth());
+
+    NamedDecl *NewParam = transformTemplateParameter(
+        SemaRef, DC, Param, Args, PartialSpecParams.size(),
+        Template->getTemplateDepth());
+    if (!NewParam)
+      return;
+
+    PartialSpecParams.push_back(NewParam);
+    SubstArgs.push_back(Context.getInjectedTemplateArg(NewParam));
+  }
+
+  Expr *RequiresClause = nullptr;
+  MultiLevelTemplateArgumentList Args;
+  Args.setKind(TemplateSubstitutionKind::Rewrite);
+  Args.addOuterTemplateArguments(SubstArgs);
+  Args.addOuterRetainedLevels(Template->getTemplateDepth());
+  if (Expr *TemplateRC = TemplateTPL->getRequiresClause()) {
+    ExprResult E = SemaRef.SubstExpr(TemplateRC, Args);
+    if (E.isInvalid())
+      return;
+    RequiresClause = E.getAs<Expr>();
+  }
+  auto *PartialSpecTPL = TemplateParameterList::Create(
+      Context, TemplateTPL->getTemplateLoc(), TemplateTPL->getLAngleLoc(),
+      PartialSpecParams, TemplateTPL->getRAngleLoc(), RequiresClause);
+
+  // [over.match.class.deduct]p1.10
+  // Let A be an alias template whose template parameter list is that of
+  // [Template] and whose defining-type-id is [BaseTSI] ...
+  auto *TransformedBase =
+      SemaRef.SubstType(BaseTSI, Args, BaseLoc, DeclarationName(), true);
+  auto *BaseAD = TypeAliasDecl::Create(
+      Context, DC, SourceLocation(), BaseLoc,
+      TransformedBase->getType().getBaseTypeIdentifier(), TransformedBase);
+  std::string AliasDeclName =
+      (Twine("__ctad_alias_") + BaseTD->getName() + "_to_" +
+       Template->getName() + "_" + Twine(BaseIdx))
+          .str();
+  IdentifierInfo *AliasIdentifier = &Context.Idents.get(AliasDeclName);
+  auto *BaseATD = TypeAliasTemplateDecl::Create(
+      Context, DC, BaseLoc, DeclarationName(AliasIdentifier), PartialSpecTPL,
+      BaseAD);
+  BaseAD->setDescribedAliasTemplate(BaseATD);
+  BaseAD->setImplicit();
+  BaseATD->setImplicit();
+
+  DC->addDecl(BaseATD);
+
+  // ... given a class template `template <typename> class CC;`
+  // whose primary template is not defined ...
+  auto *TParam = TemplateTypeParmDecl::Create(
+      Context, DC, SourceLocation(), SourceLocation(),
+      Template->getTemplateDepth(), 0, nullptr, false, false);
+  TParam->setImplicit();
+  auto *MapperTPL = TemplateParameterList::Create(
+      Context, SourceLocation(), SourceLocation(),
+      ArrayRef<NamedDecl *>(TParam), SourceLocation(), nullptr);
+
+  std::string MapperDeclName =
+      (Twine("__ctad_mapper_") + BaseTD->getName() + "_to_" +
+       Template->getName() + "_" + Twine(BaseIdx))
+          .str();
+  IdentifierInfo *MapperII = &Context.Idents.get(MapperDeclName);
+  CXXRecordDecl *MapperRD = CXXRecordDecl::Create(
+      Context, CXXRecordDecl::TagKind::Struct, DC, SourceLocation(),
+      SourceLocation(), nullptr, nullptr);
+  ClassTemplateDecl *MapperTD =
+      ClassTemplateDecl::Create(Context, DC, SourceLocation(),
+                                DeclarationName(MapperII), MapperTPL, MapperRD);
+  MapperRD->setDescribedClassTemplate(MapperTD);
+  MapperTD->setImplicit();
+  MapperRD->setImplicit();
+
+  DC->addDecl(MapperTD);
+
+  // ... and with a single partial specialization whose template parameter list
+  // is that of A with the template argument list of A ...
+  TemplateArgument AliasTA(TransformedBase->getType());
+  ArrayRef<TemplateArgument> TAL(AliasTA);
+  auto MapperTemplateName =
+      Context.getCanonicalTemplateName(TemplateName(MapperTD));
+  QualType CanonType =
+      Context.getTemplateSpecializationType(MapperTemplateName, TAL);
+
+  auto *MapperSpecialization = ClassTemplatePartialSpecializationDecl::Create(
+      Context, ClassTemplatePartialSpecializationDecl::TagKind::Struct, DC,
+      SourceLocation(), SourceLocation(), PartialSpecTPL, MapperTD, TAL,
----------------
antangelo wrote:

Are you referring to the parameters themselves being incorrect between the alias/partial specialization, or the reuse of the same TemplateParameterList instance for the two decls being incorrect?
If it's the latter, we reuse the class template's TPL when creating implicit deduction guides, is that also incorrect?

https://github.com/llvm/llvm-project/pull/98788