[clang] fix DependentTemplateSpecializationType transform

[mizvekov]

This changes the transform for DTST so it rebuilds the node if any of the template arguments changed.

This fixes a regression reported here: #133610 (comment) which was introduced by #133610

There are no release notes since the regression was never released.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Matheus Izvekov (mizvekov)

Changes

This changes the transform for DTST so rebuilds the node if any of the template arguments changed.

This fixes a regression reported here: #133610 (comment) which was introduced by #133610

There are no release notes since the regression was never released.

---

Full diff: https://github.com/llvm/llvm-project/pull/134748.diff

2 Files Affected:

• (modified) clang/lib/Sema/TreeTransform.h (+12-6)
• (modified) clang/test/SemaTemplate/dependent-names.cpp (+9)

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 57fcc4b3b3682..1e126a8875331 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -7765,17 +7765,23 @@ QualType TreeTransform<Derived>::TransformDependentTemplateSpecializationType(
   NewTemplateArgs.setLAngleLoc(TL.getLAngleLoc());
   NewTemplateArgs.setRAngleLoc(TL.getRAngleLoc());
 
-  typedef TemplateArgumentLocContainerIterator<
-  DependentTemplateSpecializationTypeLoc> ArgIterator;
-  if (getDerived().TransformTemplateArguments(ArgIterator(TL, 0),
-                                              ArgIterator(TL, TL.getNumArgs()),
-                                              NewTemplateArgs))
+  auto ArgsRange = llvm::make_range<TemplateArgumentLocContainerIterator<
+      DependentTemplateSpecializationTypeLoc>>({TL, 0}, {TL, TL.getNumArgs()});
+
+  if (getDerived().TransformTemplateArguments(ArgsRange.begin(),
+                                              ArgsRange.end(), NewTemplateArgs))
     return QualType();
+  bool TemplateArgumentsChanged = !llvm::equal(
+      ArgsRange, NewTemplateArgs.arguments(),
+      [](const TemplateArgumentLoc &A, const TemplateArgumentLoc &B) {
+        return A.getArgument().structurallyEquals(B.getArgument());
+      });
 
   const DependentTemplateStorage &DTN = T->getDependentTemplateName();
 
   QualType Result = TL.getType();
-  if (getDerived().AlwaysRebuild() || SS.getScopeRep() != DTN.getQualifier()) {
+  if (getDerived().AlwaysRebuild() || SS.getScopeRep() != DTN.getQualifier() ||
+      TemplateArgumentsChanged) {
     TemplateName Name = getDerived().RebuildTemplateName(
         SS, TL.getTemplateKeywordLoc(), DTN.getName(), TL.getTemplateNameLoc(),
         /*ObjectType=*/QualType(), /*FirstQualifierInScope=*/nullptr,
diff --git a/clang/test/SemaTemplate/dependent-names.cpp b/clang/test/SemaTemplate/dependent-names.cpp
index 92620e862fe3a..538abde3eddd5 100644
--- a/clang/test/SemaTemplate/dependent-names.cpp
+++ b/clang/test/SemaTemplate/dependent-names.cpp
@@ -458,3 +458,12 @@ namespace PR37680 {
   };
   int f(b<a> ba) { return ba.add<0>(); }
 }
+
+namespace TransformDependentTemplates {
+  template <class T> struct Test1 {
+    template <class T2>
+      using Arg = typename T::template Arg<T2>;
+    void f(Arg<void>);
+    void f(Arg<int>);
+  };
+} // namespace TransformDependentTemplates