[clang] Implement CTAD for type alias template. (#77890)

Fixes #54051

This patch implements the C++20 feature -- CTAD for alias templates (P1814R0, specified in https://eel.is/c++draft/over.match.class.deduct#3). It is an initial patch:
- it cover major pieces, thus it works for most cases;
- the big missing piece is to implement the associated constraints (over.match.class.deduct#3.3) for the synthesized deduction guides, see the FIXME in code and tests;
- Some enhancements on the TreeTransform&TemplateInstantiator to allow performing instantiation on `BuildingDeductionGuides` mode;

Author: hokein

clang/docs/ReleaseNotes.rst

@@ -91,6 +91,10 @@ C++20 Feature Support
   current module units.
   Fixes `#84002 <https://github.com/llvm/llvm-project/issues/84002>`_.
 
+- Initial support for class template argument deduction (CTAD) for type alias
+  templates (`P1814R0 <https://wg21.link/p1814r0>`_).
+  (#GH54051).
+
 C++23 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -2515,10 +2515,11 @@ def err_deduced_class_template_compound_type : Error<
   "cannot %select{form pointer to|form reference to|form array of|"
   "form function returning|use parentheses when declaring variable with}0 "
   "deduced class template specialization type">;
-def err_deduced_non_class_template_specialization_type : Error<
+def err_deduced_non_class_or_alias_template_specialization_type : Error<
   "%select{<error>|function template|variable template|alias template|"
   "template template parameter|concept|template}0 %1 requires template "
-  "arguments; argument deduction only allowed for class templates">;
+  "arguments; argument deduction only allowed for class templates or alias "
+  "templates">;
 def err_deduced_class_template_ctor_ambiguous : Error<
   "ambiguous deduction for template arguments of %0">;
 def err_deduced_class_template_ctor_no_viable : Error<
@@ -8201,6 +8202,11 @@ let CategoryName = "Lambda Issue" in {
   def warn_cxx17_compat_lambda_def_ctor_assign : Warning<
     "%select{default construction|assignment}0 of lambda is incompatible with "
     "C++ standards before C++20">, InGroup<CXXPre20Compat>, DefaultIgnore;
+
+  // C++20 class template argument deduction for alias templates.
+  def warn_cxx17_compat_ctad_for_alias_templates : Warning<
+  "class template argument deduction for alias templates is incompatible with "
+  "C++ standards before C++20">, InGroup<CXXPre20Compat>, DefaultIgnore;
 }
 
 def err_return_in_captured_stmt : Error<

clang/include/clang/Sema/Sema.h

@@ -9827,6 +9827,12 @@ class Sema final {
                           ArrayRef<TemplateArgument> TemplateArgs,
                           sema::TemplateDeductionInfo &Info);
 
+  TemplateDeductionResult DeduceTemplateArguments(
+      TemplateParameterList *TemplateParams, ArrayRef<TemplateArgument> Ps,
+      ArrayRef<TemplateArgument> As, sema::TemplateDeductionInfo &Info,
+      SmallVectorImpl<DeducedTemplateArgument> &Deduced,
+      bool NumberOfArgumentsMustMatch);
+
   TemplateDeductionResult SubstituteExplicitTemplateArguments(
       FunctionTemplateDecl *FunctionTemplate,
       TemplateArgumentListInfo &ExplicitTemplateArgs,
@@ -10378,6 +10384,9 @@ class Sema final {
     InstantiatingTemplate &operator=(const InstantiatingTemplate &) = delete;
   };
 
+  bool SubstTemplateArgument(const TemplateArgumentLoc &Input,
+                             const MultiLevelTemplateArgumentList &TemplateArgs,
+                             TemplateArgumentLoc &Output);
   bool
   SubstTemplateArguments(ArrayRef<TemplateArgumentLoc> Args,
                          const MultiLevelTemplateArgumentList &TemplateArgs,
@@ -10862,9 +10871,11 @@ class Sema final {
                                   ParmVarDecl *Param);
   void InstantiateExceptionSpec(SourceLocation PointOfInstantiation,
                                 FunctionDecl *Function);
-  FunctionDecl *InstantiateFunctionDeclaration(FunctionTemplateDecl *FTD,
-                                               const TemplateArgumentList *Args,
-                                               SourceLocation Loc);
+  FunctionDecl *InstantiateFunctionDeclaration(
+      FunctionTemplateDecl *FTD, const TemplateArgumentList *Args,
+      SourceLocation Loc,
+      CodeSynthesisContext::SynthesisKind CSC =
+          CodeSynthesisContext::ExplicitTemplateArgumentSubstitution);
   void InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
                                      FunctionDecl *Function,
                                      bool Recursive = false,

clang/lib/Sema/SemaInit.cpp

@@ -10720,13 +10720,40 @@ QualType Sema::DeduceTemplateSpecializationFromInitializer(
   if (TemplateName.isDependent())
     return SubstAutoTypeDependent(TSInfo->getType());
 
-  // We can only perform deduction for class templates.
+  // We can only perform deduction for class templates or alias templates.
   auto *Template =
       dyn_cast_or_null<ClassTemplateDecl>(TemplateName.getAsTemplateDecl());
+  TemplateDecl *LookupTemplateDecl = Template;
+  if (!Template) {
+    if (auto *AliasTemplate = dyn_cast_or_null<TypeAliasTemplateDecl>(
+            TemplateName.getAsTemplateDecl())) {
+      Diag(Kind.getLocation(),
+           diag::warn_cxx17_compat_ctad_for_alias_templates);
+      LookupTemplateDecl = AliasTemplate;
+      auto UnderlyingType = AliasTemplate->getTemplatedDecl()
+                                ->getUnderlyingType()
+                                .getCanonicalType();
+      // C++ [over.match.class.deduct#3]: ..., the defining-type-id of A must be
+      // of the form
+      //   [typename] [nested-name-specifier] [template] simple-template-id
+      if (const auto *TST =
+              UnderlyingType->getAs<TemplateSpecializationType>()) {
+        Template = dyn_cast_or_null<ClassTemplateDecl>(
+            TST->getTemplateName().getAsTemplateDecl());
+      } else if (const auto *RT = UnderlyingType->getAs<RecordType>()) {
+        // Cases where template arguments in the RHS of the alias are not
+        // dependent. e.g.
+        //   using AliasFoo = Foo<bool>;
+        if (const auto *CTSD = llvm::dyn_cast<ClassTemplateSpecializationDecl>(
+                RT->getAsCXXRecordDecl()))
+          Template = CTSD->getSpecializedTemplate();
+      }
+    }
+  }
   if (!Template) {
     Diag(Kind.getLocation(),
-         diag::err_deduced_non_class_template_specialization_type)
-      << (int)getTemplateNameKindForDiagnostics(TemplateName) << TemplateName;
+         diag::err_deduced_non_class_or_alias_template_specialization_type)
+        << (int)getTemplateNameKindForDiagnostics(TemplateName) << TemplateName;
     if (auto *TD = TemplateName.getAsTemplateDecl())
       NoteTemplateLocation(*TD);
     return QualType();
@@ -10753,10 +10780,10 @@ QualType Sema::DeduceTemplateSpecializationFromInitializer(
   //     template-name, a function template [...]
   //  - For each deduction-guide, a function or function template [...]
   DeclarationNameInfo NameInfo(
-      Context.DeclarationNames.getCXXDeductionGuideName(Template),
+      Context.DeclarationNames.getCXXDeductionGuideName(LookupTemplateDecl),
       TSInfo->getTypeLoc().getEndLoc());
   LookupResult Guides(*this, NameInfo, LookupOrdinaryName);
-  LookupQualifiedName(Guides, Template->getDeclContext());
+  LookupQualifiedName(Guides, LookupTemplateDecl->getDeclContext());
 
   // FIXME: Do not diagnose inaccessible deduction guides. The standard isn't
   // clear on this, but they're not found by name so access does not apply.