[clang] Implement CWG2398 provisional TTP matching to class templates

This extends default argument deduction to cover class templates as
well.

This solves some ambuguity introduced in P0522 regarding how
template template parameters are partially ordered, and should reduce
the negative impact of enabling `-frelaxed-template-template-args`
by default.

Given the following example:
```C++
template <class T1, class T2 = float> struct A;
template <class T3> struct B;

template <template <class T4> class TT1, class T5> struct B<TT1<T5>>;   // #1
template <class T6, class T7>                      struct B<A<T6, T7>>; // #2

template struct B<A<int>>;
```
Prior to P0522, `#2` was picked. Afterwards, this became ambiguous.
This patch restores the pre-P0522 behavior, `#2` is picked again.

clang-tools-extra/clangd/DumpAST.cpp

@@ -187,6 +187,7 @@ class DumpVisitor : public RecursiveASTVisitor<DumpVisitor> {
       TEMPLATE_KIND(SubstTemplateTemplateParm);
       TEMPLATE_KIND(SubstTemplateTemplateParmPack);
       TEMPLATE_KIND(UsingTemplate);
+      TEMPLATE_KIND(DeducedTemplate);
 #undef TEMPLATE_KIND
     }
     llvm_unreachable("Unhandled NameKind enum");

clang-tools-extra/clangd/SemanticHighlighting.cpp

@@ -1120,6 +1120,7 @@ class CollectExtraHighlightings
     case TemplateName::SubstTemplateTemplateParm:
     case TemplateName::SubstTemplateTemplateParmPack:
     case TemplateName::UsingTemplate:
+    case TemplateName::DeducedTemplate:
       // Names that could be resolved to a TemplateDecl are handled elsewhere.
       break;
     }

clang/docs/ReleaseNotes.rst

@@ -149,6 +149,9 @@ Resolutions to C++ Defect Reports
   of the target type, even if the type of the bit-field is larger.
   (`CWG2627: Bit-fields and narrowing conversions <https://cplusplus.github.io/CWG/issues/2627.html>`_)
 
+- Clang now has improved resolution to CWG2398, allowing class templates to have
+  default arguments deduced when partial ordering.
+
 C Language Changes
 ------------------
 

clang/include/clang/AST/ASTContext.h

@@ -264,6 +264,8 @@ class ASTContext : public RefCountedBase<ASTContext> {
   mutable llvm::ContextualFoldingSet<SubstTemplateTemplateParmPackStorage,
                                      ASTContext&>
     SubstTemplateTemplateParmPacks;
+  mutable llvm::ContextualFoldingSet<DeducedTemplateStorage, ASTContext &>
+      DeducedTemplates;
 
   mutable llvm::ContextualFoldingSet<ArrayParameterType, ASTContext &>
       ArrayParameterTypes;
@@ -2291,6 +2293,9 @@ class ASTContext : public RefCountedBase<ASTContext> {
                                                 unsigned Index,
                                                 bool Final) const;
 
+  TemplateName getDeducedTemplateName(TemplateName Underlying,
+                                      DefaultArguments DefaultArgs) const;
+
   enum GetBuiltinTypeError {
     /// No error
     GE_None,
@@ -2774,11 +2779,13 @@ class ASTContext : public RefCountedBase<ASTContext> {
   /// template name uses the shortest form of the dependent
   /// nested-name-specifier, which itself contains all canonical
   /// types, values, and templates.
-  TemplateName getCanonicalTemplateName(const TemplateName &Name) const;
+  TemplateName getCanonicalTemplateName(TemplateName Name,
+                                        bool IgnoreDeduced = false) const;
 
   /// Determine whether the given template names refer to the same
   /// template.
-  bool hasSameTemplateName(const TemplateName &X, const TemplateName &Y) const;
+  bool hasSameTemplateName(const TemplateName &X, const TemplateName &Y,
+                           bool IgnoreDeduced = false) const;
 
   /// Determine whether the two declarations refer to the same entity.
   bool isSameEntity(const NamedDecl *X, const NamedDecl *Y) const;

clang/include/clang/AST/ASTImporter.h

@@ -484,6 +484,11 @@ class TypeSourceInfo;
     /// the declarations it contains.
     [[nodiscard]] llvm::Error ImportDefinition(Decl *From);
 
+    llvm::Error
+    ImportTemplateArguments(ArrayRef<TemplateArgument> FromArgs,
+                            SmallVectorImpl<TemplateArgument> &ToArgs);
+    Expected<TemplateArgument> Import(const TemplateArgument &From);
+
     /// Cope with a name conflict when importing a declaration into the
     /// given context.
     ///

clang/include/clang/AST/DependenceFlags.h

@@ -315,6 +315,11 @@ toTemplateNameDependence(NestedNameSpecifierDependence D) {
   return Dependence(D).templateName();
 }
 
+inline TemplateNameDependence
+toTemplateNameDependence(TemplateArgumentDependence D) {
+  return Dependence(D).templateName();
+}
+
 LLVM_ENABLE_BITMASK_ENUMS_IN_NAMESPACE();
 
 } // namespace clang

clang/include/clang/AST/PropertiesBase.td

@@ -752,6 +752,23 @@ let Class = PropertyTypeCase<TemplateName, "SubstTemplateTemplateParmPack"> in {
     return ctx.getSubstTemplateTemplateParmPack(argumentPack, associatedDecl, index, final);
   }]>;
 }
+let Class = PropertyTypeCase<TemplateName, "DeducedTemplate"> in {
+  def : ReadHelper<[{
+    auto DTS = node.getAsDeducedTemplateName();
+  }]>;
+  def : Property<"underlying", TemplateName> {
+    let Read = [{ DTS->getUnderlying() }];
+  }
+  def : Property<"startPos", UInt32> {
+    let Read = [{ DTS->getDefaultArguments().StartPos }];
+  }
+  def : Property<"defaultArgs", Array<TemplateArgument>> {
+    let Read = [{ DTS->getDefaultArguments().Args }];
+  }
+  def : Creator<[{
+    return ctx.getDeducedTemplateName(underlying, {startPos, defaultArgs});
+  }]>;
+}
 
 // Type cases for TemplateArgument.
 def : PropertyTypeKind<TemplateArgument, TemplateArgumentKind,

clang/include/clang/AST/TemplateName.h

@@ -34,6 +34,7 @@ class NestedNameSpecifier;
 enum OverloadedOperatorKind : int;
 class OverloadedTemplateStorage;
 class AssumedTemplateStorage;
+class DeducedTemplateStorage;
 struct PrintingPolicy;
 class QualifiedTemplateName;
 class SubstTemplateTemplateParmPackStorage;
@@ -50,16 +51,17 @@ class UncommonTemplateNameStorage {
   enum Kind {
     Overloaded,
     Assumed, // defined in DeclarationName.h
+    Deduced,
     SubstTemplateTemplateParm,
     SubstTemplateTemplateParmPack
   };
 
   struct BitsTag {
     LLVM_PREFERRED_TYPE(Kind)
-    unsigned Kind : 2;
+    unsigned Kind : 3;
 
     // The template parameter index.
-    unsigned Index : 15;
+    unsigned Index : 14;
 
     /// The pack index, or the number of stored templates
     /// or template arguments, depending on which subclass we have.
@@ -90,6 +92,12 @@ class UncommonTemplateNameStorage {
              : nullptr;
   }
 
+  DeducedTemplateStorage *getAsDeducedTemplateName() {
+    return Bits.Kind == Deduced
+               ? reinterpret_cast<DeducedTemplateStorage *>(this)
+               : nullptr;
+  }
+
   SubstTemplateTemplateParmStorage *getAsSubstTemplateTemplateParm() {
     return Bits.Kind == SubstTemplateTemplateParm
              ? reinterpret_cast<SubstTemplateTemplateParmStorage *>(this)
@@ -172,6 +180,15 @@ class SubstTemplateTemplateParmPackStorage : public UncommonTemplateNameStorage,
                       unsigned Index, bool Final);
 };
 
+struct DefaultArguments {
+  // The position in the template parameter list
+  // the first argument corresponds to.
+  unsigned StartPos;
+  ArrayRef<TemplateArgument> Args;
+
+  operator bool() const { return !Args.empty(); }
+};
+
 /// Represents a C++ template name within the type system.
 ///
 /// A C++ template name refers to a template within the C++ type
@@ -246,6 +263,10 @@ class TemplateName {
     /// A template name that refers to a template declaration found through a
     /// specific using shadow declaration.
     UsingTemplate,
+
+    /// A template name that refers to another TemplateName with deduced default
+    /// arguments.
+    DeducedTemplate,
   };
 
   TemplateName() = default;
@@ -257,6 +278,7 @@ class TemplateName {
   explicit TemplateName(QualifiedTemplateName *Qual);
   explicit TemplateName(DependentTemplateName *Dep);
   explicit TemplateName(UsingShadowDecl *Using);
+  explicit TemplateName(DeducedTemplateStorage *Deduced);
 
   /// Determine whether this template name is NULL.
   bool isNull() const;
@@ -271,7 +293,13 @@ class TemplateName {
   /// to, if any. If the template name does not refer to a specific
   /// declaration because it is a dependent name, or if it refers to a
   /// set of function templates, returns NULL.
-  TemplateDecl *getAsTemplateDecl() const;
+  TemplateDecl *getAsTemplateDecl(bool IgnoreDeduced = false) const;
+
+  /// Retrieves the underlying template declaration that
+  /// this template name refers to, along with the
+  /// deduced default arguments, if any.
+  std::pair<TemplateDecl *, DefaultArguments>
+  getTemplateDeclAndDefaultArgs() const;
 
   /// Retrieve the underlying, overloaded function template
   /// declarations that this template name refers to, if known.
@@ -313,6 +341,11 @@ class TemplateName {
   /// template declaration is introduced, if any.
   UsingShadowDecl *getAsUsingShadowDecl() const;
 
+  /// Retrieve the deduced template info, if any.
+  DeducedTemplateStorage *getAsDeducedTemplateName() const;
+
+  std::optional<TemplateName> desugar(bool IgnoreDeduced) const;
+
   TemplateName getUnderlying() const;
 
   TemplateNameDependence getDependence() const;
@@ -412,6 +445,30 @@ class SubstTemplateTemplateParmStorage
                       std::optional<unsigned> PackIndex);
 };
 
+class DeducedTemplateStorage : public UncommonTemplateNameStorage,
+                               public llvm::FoldingSetNode {
+  friend class ASTContext;
+
+  TemplateName Underlying;
+
+  DeducedTemplateStorage(TemplateName Underlying,
+                         const DefaultArguments &DefArgs);
+
+public:
+  TemplateName getUnderlying() const { return Underlying; }
+
+  DefaultArguments getDefaultArguments() const {
+    return {/*StartPos=*/Bits.Index,
+            /*Args=*/{reinterpret_cast<const TemplateArgument *>(this + 1),
+                      Bits.Data}};
+  }
+
+  void Profile(llvm::FoldingSetNodeID &ID, const ASTContext &Context) const;
+
+  static void Profile(llvm::FoldingSetNodeID &ID, const ASTContext &Context,
+                      TemplateName Underlying, const DefaultArguments &DefArgs);
+};
+
 inline TemplateName TemplateName::getUnderlying() const {
   if (SubstTemplateTemplateParmStorage *subst
         = getAsSubstTemplateTemplateParm())

clang/include/clang/Sema/Sema.h

@@ -11725,6 +11725,9 @@ class Sema final : public SemaBase {
   /// receive true if the cause for the error is the associated constraints of
   /// the template not being satisfied by the template arguments.
   ///
+  /// \param DefaultArgs any default arguments from template specialization
+  /// deduction.
+  ///
   /// \param PartialOrderingTTP If true, assume these template arguments are
   /// the injected template arguments for a template template parameter.
   /// This will relax the requirement that all its possible uses are valid:
@@ -11734,7 +11737,8 @@ class Sema final : public SemaBase {
   /// \returns true if an error occurred, false otherwise.
   bool CheckTemplateArgumentList(
       TemplateDecl *Template, SourceLocation TemplateLoc,
-      TemplateArgumentListInfo &TemplateArgs, bool PartialTemplateArgs,
+      TemplateArgumentListInfo &TemplateArgs,
+      const DefaultArguments &DefaultArgs, bool PartialTemplateArgs,
       SmallVectorImpl<TemplateArgument> &SugaredConverted,
       SmallVectorImpl<TemplateArgument> &CanonicalConverted,
       bool UpdateArgsWithConversions = true,
@@ -12471,8 +12475,8 @@ class Sema final : public SemaBase {
                                     sema::TemplateDeductionInfo &Info);
 
   bool isTemplateTemplateParameterAtLeastAsSpecializedAs(
-      TemplateParameterList *PParam, TemplateDecl *AArg, SourceLocation Loc,
-      bool IsDeduced);
+      TemplateParameterList *PParam, TemplateDecl *AArg,
+      const DefaultArguments &DefaultArgs, SourceLocation Loc, bool IsDeduced);
 
   /// Mark which template parameters are used in a given expression.
   ///