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

[via cfe-commits]

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@@ -2612,44 +2671,309 @@ struct ConvertConstructorToDeductionGuideTransform {
     SemaRef.CurrentInstantiationScope->InstantiatedLocal(OldParam, NewParam);
     return NewParam;
   }
+};
 
-  FunctionTemplateDecl *buildDeductionGuide(
-      TemplateParameterList *TemplateParams, CXXConstructorDecl *Ctor,
-      ExplicitSpecifier ES, TypeSourceInfo *TInfo, SourceLocation LocStart,
-      SourceLocation Loc, SourceLocation LocEnd,
-      llvm::ArrayRef<TypedefNameDecl *> MaterializedTypedefs = {}) {
-    DeclarationNameInfo Name(DeductionGuideName, Loc);
-    ArrayRef<ParmVarDecl *> Params =
-        TInfo->getTypeLoc().castAs<FunctionProtoTypeLoc>().getParams();
+// Find all template parameters of the AliasTemplate that appear in the
+// given DeducedArgs.
+SmallVector<unsigned>
+FindAppearedTemplateParamsInAlias(ArrayRef<TemplateArgument> DeducedArgs,
+                                  TypeAliasTemplateDecl *AliasTemplate) {
+  struct FindAppearedTemplateParams
+      : public RecursiveASTVisitor<FindAppearedTemplateParams> {
+    llvm::DenseSet<NamedDecl *> TemplateParamsInAlias;
+    llvm::DenseSet<const NamedDecl *> AppearedTemplateParams;
+
+    FindAppearedTemplateParams(ArrayRef<NamedDecl *> TemplateParamsInAlias)
+        : TemplateParamsInAlias(TemplateParamsInAlias.begin(),
+                                TemplateParamsInAlias.end()) {}
+
+    bool VisitTemplateTypeParmType(TemplateTypeParmType *TTP) {
+      TTP->getIndex();
+      MarkAppeared(TTP->getDecl());
+      return true;
+    }
+    bool VisitDeclRefExpr(DeclRefExpr *DRE) {
+      MarkAppeared(DRE->getFoundDecl());
+      return true;
+    }
 
-    // Build the implicit deduction guide template.
-    auto *Guide =
-        CXXDeductionGuideDecl::Create(SemaRef.Context, DC, LocStart, ES, Name,
-                                      TInfo->getType(), TInfo, LocEnd, Ctor);
-    Guide->setImplicit();
-    Guide->setParams(Params);
+    void MarkAppeared(NamedDecl *ND) {
+      if (TemplateParamsInAlias.contains(ND))
+        AppearedTemplateParams.insert(ND);
+    }
+  };
+  ArrayRef<NamedDecl *> TemplateParamsInAlias =
+      AliasTemplate->getTemplateParameters()->asArray();
+  FindAppearedTemplateParams MarkAppeared(TemplateParamsInAlias);
+  MarkAppeared.TraverseTemplateArguments(DeducedArgs);
 
-    for (auto *Param : Params)
-      Param->setDeclContext(Guide);
-    for (auto *TD : MaterializedTypedefs)
-      TD->setDeclContext(Guide);
+  SmallVector<unsigned> Results;
+  for (unsigned Index = 0; Index < TemplateParamsInAlias.size(); ++Index) {
+    if (MarkAppeared.AppearedTemplateParams.contains(
+            TemplateParamsInAlias[Index]))
+      Results.push_back(Index);
+  }
+  return Results;
+}
 
-    auto *GuideTemplate = FunctionTemplateDecl::Create(
-        SemaRef.Context, DC, Loc, DeductionGuideName, TemplateParams, Guide);
-    GuideTemplate->setImplicit();
-    Guide->setDescribedFunctionTemplate(GuideTemplate);
+bool hasDeclaredDeductionGuides(DeclarationName Name, DeclContext* DC) {
+  // Check whether we've already declared deduction guides for this template.
+  // FIXME: Consider storing a flag on the template to indicate this.
+  auto Existing = DC->lookup(Name);
+  for (auto *D : Existing)
+    if (D->isImplicit())
+      return true;
+  return false;
+}
 
-    if (isa<CXXRecordDecl>(DC)) {
-      Guide->setAccess(AS_public);
-      GuideTemplate->setAccess(AS_public);
+// Build deduction guides for a type alias template.
+void DeclareImplicitDeductionGuidesForTypeAlias(
+    Sema &SemaRef, TypeAliasTemplateDecl *AliasTemplate, SourceLocation Loc) {
+  auto &Context = SemaRef.Context;
+  // FIXME: if there is an explicit deduction guide after the first use of the
+  // type alias usage, we will not cover this explicit deduction guide. fix this
+  // case.
+  if (hasDeclaredDeductionGuides(
+          Context.DeclarationNames.getCXXDeductionGuideName(AliasTemplate),
+          AliasTemplate->getDeclContext()))
+    return;
+  // Unrap the sugar ElaboratedType.
+  auto RhsType = AliasTemplate->getTemplatedDecl()
+                     ->getUnderlyingType()
+                     .getSingleStepDesugaredType(Context);
+  TemplateDecl *Template = nullptr;
+  llvm::ArrayRef<TemplateArgument> AliasRhsTemplateArgs;
+  if (const auto *TST = RhsType->getAs<TemplateSpecializationType>()) {
+    // TemplateName in TEST can be a TypeAliasTemplateDecl if
----------------
cor3ntin wrote:

```suggestion
    // TemplateName in TST can be a TypeAliasTemplateDecl if
```

https://github.com/llvm/llvm-project/pull/77890