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

[Haojian Wu via cfe-commits]

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@@ -10598,10 +10598,34 @@ 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 && getLangOpts().CPlusPlus20) { // type alias template
+    if (auto *AliasTemplate = dyn_cast_or_null<TypeAliasTemplateDecl>(
+            TemplateName.getAsTemplateDecl());
+        AliasTemplate) {
+      LookupTemplateDecl = AliasTemplate;
+      auto UnderlyingType = AliasTemplate->getTemplatedDecl()
+                                ->getUnderlyingType()
+                                .getDesugaredType(Context);
+      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();
+      }
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hokein wrote:

ah, sorry, I added it and then removed later. There are some other cases which are not supported, see the testcase `test17`. 

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