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

[Haojian Wu 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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hokein wrote:

We could add one for that, but I'm not sure, it seems that in general clang doesn't provide diagnostics for features that aren't available in the current `-std=` version (at least I didn't such a diagnostic).

For example, when we use C++ concepts without `-std=c++20`, clang rejects it with a diagnostic `unknown type name 'concept'`. gcc have better diagnostics (`note: 'concept' only available with '-std=c++20' or '-fconcepts'`).

I have a somewhat-relevant FIXME below, it is about to adjust `err_deduced_non_class_template_specialization_type` message to include the type-alias templates.

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