[clang] [Clang][C++26] Implement P3865R1 - CTAD for type template template parameters (PR #191409)

[Oliver Hunt via cfe-commits]

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@@ -1604,3 +1604,60 @@ void Sema::DeclareImplicitDeductionGuides(TemplateDecl *Template,
 
   SavedContext.pop();
 }
+
+TypeAliasTemplateDecl *Sema::BuildAliasForCTADFromTypeTemplateParameter(
+    TemplateTemplateParmDecl *D, TemplateName Replacement, SourceLocation Loc) {
+
+  // [C++26] [over.match.class.deduct]p3
+  // When resolving a placeholder for a deduced class type where the
+  // template-name designates a type template template parameter P, let A be an
+  // alias template whose template parameter list is that of P and whose
+  // defining-type-id designates the type template template argument with a
+  // simpletemplate-id in which the template-argument-list consists of a list of
+  // identifiers naming each template-parameter of P, with the argument being a
+  // pack expansion if the template-parameter is a pack. A is then used instead
+  // of the original template-name to resolve the placeholder
+
+  LocalInstantiationScope Scope(SemaRef);
+
+  auto &AST = SemaRef.getASTContext();
+  auto *Ctx = CurContext->getParent();
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ojhunt wrote:

nit: I think this should be DeclContext* rather than auto, and I'd prefer Ctx be given a clearer name -ParentCtx would seem reasonable?

That would also make it more reasonable to use Context or Ctx for the local ASTContext reference

https://github.com/llvm/llvm-project/pull/191409