[clang] [clang] CTAD: fix the aggregate deduction guide for alias templates. (PR #90894)

[Haojian Wu via cfe-commits]

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@@ -2803,7 +2803,207 @@ getRHSTemplateDeclAndArgs(Sema &SemaRef, TypeAliasTemplateDecl *AliasTemplate) {
   return {Template, AliasRhsTemplateArgs};
 }
 
-// Build deduction guides for a type alias template.
+// Build deduction guides for a type alias template from the given underlying
+// deduction guide F.
+FunctionTemplateDecl *
+BuildDeductionGuideForTypeAlias(Sema &SemaRef,
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hokein wrote:

> to clarify, in the actual fix you removed a whole bunch of code?
https://github.com/llvm/llvm-project/commit/621899224aa7d59ab70675f6ff32b298b06a515a

Yes, that's correct. The removed code is an incorrect implementation which will generate wrong deduction guides.

https://github.com/llvm/llvm-project/pull/90894