[Clang] prevent errors for deduction guides using deduced type aliases

clang/docs/ReleaseNotes.rst

@@ -583,6 +583,8 @@ Improvements to Clang's diagnostics
 - For an rvalue reference bound to a temporary struct with an integer member, Clang will detect constant integer overflow
   in the initializer for the integer member (#GH46755).
 
+- Clang now prevents errors for deduction guides with deduced type aliases (#GH54909).
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/lib/Sema/SemaDeclCXX.cpp

@@ -11451,7 +11451,11 @@ bool Sema::CheckDeductionGuideDeclarator(Declarator &D, QualType &R,
     bool MightInstantiateToSpecialization = false;
     if (auto RetTST =
             TSI->getTypeLoc().getAsAdjusted<TemplateSpecializationTypeLoc>()) {
-      TemplateName SpecifiedName = RetTST.getTypePtr()->getTemplateName();
+      const TemplateSpecializationType *TST = RetTST.getTypePtr();
+      while (TST->isTypeAlias())
+        TST = TST->getAliasedType()->getAs<TemplateSpecializationType>();
+
+      TemplateName SpecifiedName = TST->getTemplateName();
       bool TemplateMatches = Context.hasSameTemplateName(
           SpecifiedName, GuidedTemplate, /*IgnoreDeduced=*/true);
 

clang/test/CXX/temp/temp.deduct.guide/p3.cpp

@@ -33,7 +33,7 @@ template<template<typename> typename TT> struct E { // expected-note 2{{template
 };
 
 A(int) -> int; // expected-error {{deduced type 'int' of deduction guide is not a specialization of template 'A'}}
-template <typename T> A(T)->B<T>;         // expected-error {{deduced type 'B<T>' (aka 'A<T>') of deduction guide is not written as a specialization of template 'A'}}
+template <typename T> A(T)->B<T>;
 template<typename T> A(T*) -> const A<T>; // expected-error {{deduced type 'const A<T>' of deduction guide is not a specialization of template 'A'}}
 
 // A deduction-guide shall be declared in the same scope as the corresponding
@@ -71,3 +71,13 @@ namespace WrongScope {
     Local(int) -> Local<int>; // expected-error {{expected}}
   }
 }
+
+namespace GH54909 {
+template <typename T> struct A {};
+A(void) -> A<int>;
+
+template <typename T> using B = A<T>;
+template <typename T> using C = B<T>;
+template <typename T> using D = C<T>;
+template <typename T> A(T) -> D<T>;
+}