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

[Erich Keane via cfe-commits]

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@@ -10598,10 +10598,36 @@ 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
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erichkeane wrote:

I think we need a diagnostic here for the 'is a C++20 feature', right?

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