[Clang][Sema] Substitute for the type aliases inside of a CTAD guide (#94740)

Similar to the approach of handling nested class templates when building
a CTAD guide, we substitute the template parameters of a type alias
declaration with the instantiating template arguments in order to ensure
the guide eventually doesn't reference any outer template parameters.

For example,
```cpp
template <class T> struct Outer {
  using Alias = S<T>;
  template <class U> struct Inner {
    Inner(Alias);
  };
};
```
we used to retain the reference to T accidentally because the
TreeTransform does nothing on type alias Decls by default.

Fixes https://github.com/llvm/llvm-project/issues/94614

clang/docs/ReleaseNotes.rst

@@ -1000,6 +1000,7 @@ Bug Fixes to C++ Support
   evaluated to an integer. (#GH96670).
 - Fixed a bug where references to lambda capture inside a ``noexcept`` specifier were not correctly
   instantiated. (#GH95735).
+- Fixed a CTAD substitution bug involving type aliases that reference outer template parameters. (#GH94614).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Sema/Template.h

@@ -711,6 +711,7 @@ enum class TemplateSubstitutionKind : char {
         VarTemplateSpecializationDecl *PrevDecl = nullptr);
 
     Decl *InstantiateTypedefNameDecl(TypedefNameDecl *D, bool IsTypeAlias);
+    Decl *InstantiateTypeAliasTemplateDecl(TypeAliasTemplateDecl *D);
     ClassTemplatePartialSpecializationDecl *
     InstantiateClassTemplatePartialSpecialization(
                                               ClassTemplateDecl *ClassTemplate,

clang/lib/Sema/SemaTemplate.cpp

@@ -2178,23 +2178,110 @@ namespace {
 class ExtractTypeForDeductionGuide
   : public TreeTransform<ExtractTypeForDeductionGuide> {
   llvm::SmallVectorImpl<TypedefNameDecl *> &MaterializedTypedefs;
+  ClassTemplateDecl *NestedPattern;
+  const MultiLevelTemplateArgumentList *OuterInstantiationArgs;
+  std::optional<TemplateDeclInstantiator> TypedefNameInstantiator;
 
 public:
   typedef TreeTransform<ExtractTypeForDeductionGuide> Base;
   ExtractTypeForDeductionGuide(
       Sema &SemaRef,
-      llvm::SmallVectorImpl<TypedefNameDecl *> &MaterializedTypedefs)
-      : Base(SemaRef), MaterializedTypedefs(MaterializedTypedefs) {}
+      llvm::SmallVectorImpl<TypedefNameDecl *> &MaterializedTypedefs,
+      ClassTemplateDecl *NestedPattern,
+      const MultiLevelTemplateArgumentList *OuterInstantiationArgs)
+      : Base(SemaRef), MaterializedTypedefs(MaterializedTypedefs),
+        NestedPattern(NestedPattern),
+        OuterInstantiationArgs(OuterInstantiationArgs) {
+    if (OuterInstantiationArgs)
+      TypedefNameInstantiator.emplace(
+          SemaRef, SemaRef.getASTContext().getTranslationUnitDecl(),
+          *OuterInstantiationArgs);
+  }
 
   TypeSourceInfo *transform(TypeSourceInfo *TSI) { return TransformType(TSI); }
 
+  /// Returns true if it's safe to substitute \p Typedef with
+  /// \p OuterInstantiationArgs.
+  bool mightReferToOuterTemplateParameters(TypedefNameDecl *Typedef) {
+    if (!NestedPattern)
+      return false;
+
+    static auto WalkUp = [](DeclContext *DC, DeclContext *TargetDC) {
+      if (DC->Equals(TargetDC))
+        return true;
+      while (DC->isRecord()) {
+        if (DC->Equals(TargetDC))
+          return true;
+        DC = DC->getParent();
+      }
+      return false;
+    };
+
+    if (WalkUp(Typedef->getDeclContext(), NestedPattern->getTemplatedDecl()))
+      return true;
+    if (WalkUp(NestedPattern->getTemplatedDecl(), Typedef->getDeclContext()))
+      return true;
+    return false;
+  }
+
+  QualType
+  RebuildTemplateSpecializationType(TemplateName Template,
+                                    SourceLocation TemplateNameLoc,
+                                    TemplateArgumentListInfo &TemplateArgs) {
+    if (!OuterInstantiationArgs ||
+        !isa_and_present<TypeAliasTemplateDecl>(Template.getAsTemplateDecl()))
+      return Base::RebuildTemplateSpecializationType(Template, TemplateNameLoc,
+                                                     TemplateArgs);
+
+    auto *TATD = cast<TypeAliasTemplateDecl>(Template.getAsTemplateDecl());
+    auto *Pattern = TATD;
+    while (Pattern->getInstantiatedFromMemberTemplate())
+      Pattern = Pattern->getInstantiatedFromMemberTemplate();
+    if (!mightReferToOuterTemplateParameters(Pattern->getTemplatedDecl()))
+      return Base::RebuildTemplateSpecializationType(Template, TemplateNameLoc,
+                                                     TemplateArgs);
+
+    Decl *NewD =
+        TypedefNameInstantiator->InstantiateTypeAliasTemplateDecl(TATD);
+    if (!NewD)
+      return QualType();
+
+    auto *NewTATD = cast<TypeAliasTemplateDecl>(NewD);
+    MaterializedTypedefs.push_back(NewTATD->getTemplatedDecl());
+
+    return Base::RebuildTemplateSpecializationType(
+        TemplateName(NewTATD), TemplateNameLoc, TemplateArgs);
+  }
+
   QualType TransformTypedefType(TypeLocBuilder &TLB, TypedefTypeLoc TL) {
     ASTContext &Context = SemaRef.getASTContext();
     TypedefNameDecl *OrigDecl = TL.getTypedefNameDecl();
     TypedefNameDecl *Decl = OrigDecl;
     // Transform the underlying type of the typedef and clone the Decl only if
     // the typedef has a dependent context.
-    if (OrigDecl->getDeclContext()->isDependentContext()) {
+    bool InDependentContext = OrigDecl->getDeclContext()->isDependentContext();
+
+    // A typedef/alias Decl within the NestedPattern may reference the outer
+    // template parameters. They're substituted with corresponding instantiation
+    // arguments here and in RebuildTemplateSpecializationType() above.
+    // Otherwise, we would have a CTAD guide with "dangling" template
+    // parameters.
+    // For example,
+    //   template <class T> struct Outer {
+    //     using Alias = S<T>;
+    //     template <class U> struct Inner {
+    //       Inner(Alias);
+    //     };
+    //   };
+    if (OuterInstantiationArgs && InDependentContext &&
+        TL.getTypePtr()->isInstantiationDependentType()) {
+      Decl = cast_if_present<TypedefNameDecl>(
+          TypedefNameInstantiator->InstantiateTypedefNameDecl(
+              OrigDecl, /*IsTypeAlias=*/isa<TypeAliasDecl>(OrigDecl)));
+      if (!Decl)
+        return QualType();
+      MaterializedTypedefs.push_back(Decl);
+    } else if (InDependentContext) {
       TypeLocBuilder InnerTLB;
       QualType Transformed =
           TransformType(InnerTLB, OrigDecl->getTypeSourceInfo()->getTypeLoc());
@@ -2541,8 +2628,9 @@ private:
       // defined outside of the surrounding class template. That is T in the
       // above example.
       if (NestedPattern) {
-        NewParam = transformFunctionTypeParam(NewParam, OuterInstantiationArgs,
-                                              MaterializedTypedefs);
+        NewParam = transformFunctionTypeParam(
+            NewParam, OuterInstantiationArgs, MaterializedTypedefs,
+            /*TransformingOuterPatterns=*/true);
         if (!NewParam)
           return QualType();
       }
@@ -2550,7 +2638,8 @@ private:
       // defined at the class template and the constructor. In this example,
       // they're U and V, respectively.
       NewParam =
-          transformFunctionTypeParam(NewParam, Args, MaterializedTypedefs);
+          transformFunctionTypeParam(NewParam, Args, MaterializedTypedefs,
+                                     /*TransformingOuterPatterns=*/false);
       if (!NewParam)
         return QualType();
       ParamTypes.push_back(NewParam->getType());
@@ -2594,7 +2683,8 @@ private:
 
   ParmVarDecl *transformFunctionTypeParam(
       ParmVarDecl *OldParam, MultiLevelTemplateArgumentList &Args,
-      llvm::SmallVectorImpl<TypedefNameDecl *> &MaterializedTypedefs) {
+      llvm::SmallVectorImpl<TypedefNameDecl *> &MaterializedTypedefs,
+      bool TransformingOuterPatterns) {
     TypeSourceInfo *OldDI = OldParam->getTypeSourceInfo();
     TypeSourceInfo *NewDI;
     if (auto PackTL = OldDI->getTypeLoc().getAs<PackExpansionTypeLoc>()) {
@@ -2617,7 +2707,9 @@ private:
     // members of the current instantiations with the definitions of those
     // typedefs, avoiding triggering instantiation of the deduced type during
     // deduction.
-    NewDI = ExtractTypeForDeductionGuide(SemaRef, MaterializedTypedefs)
+    NewDI = ExtractTypeForDeductionGuide(
+                SemaRef, MaterializedTypedefs, NestedPattern,
+                TransformingOuterPatterns ? &Args : nullptr)
                 .transform(NewDI);
 
     // Resolving a wording defect, we also inherit default arguments from the

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -1096,8 +1096,8 @@ Decl *TemplateDeclInstantiator::VisitTypeAliasDecl(TypeAliasDecl *D) {
   return Typedef;
 }
 
-Decl *
-TemplateDeclInstantiator::VisitTypeAliasTemplateDecl(TypeAliasTemplateDecl *D) {
+Decl *TemplateDeclInstantiator::InstantiateTypeAliasTemplateDecl(
+    TypeAliasTemplateDecl *D) {
   // Create a local instantiation scope for this type alias template, which
   // will contain the instantiations of the template parameters.
   LocalInstantiationScope Scope(SemaRef);
@@ -1143,7 +1143,14 @@ TemplateDeclInstantiator::VisitTypeAliasTemplateDecl(TypeAliasTemplateDecl *D) {
   if (!PrevAliasTemplate)
     Inst->setInstantiatedFromMemberTemplate(D);
 
-  Owner->addDecl(Inst);
+  return Inst;
+}
+
+Decl *
+TemplateDeclInstantiator::VisitTypeAliasTemplateDecl(TypeAliasTemplateDecl *D) {
+  Decl *Inst = InstantiateTypeAliasTemplateDecl(D);
+  if (Inst)
+    Owner->addDecl(Inst);
 
   return Inst;
 }

clang/test/SemaTemplate/nested-deduction-guides.cpp

@@ -1,5 +1,4 @@
 // RUN: %clang_cc1 -std=c++17 -verify %s
-// expected-no-diagnostics
 
 template<typename T> struct A {
   template<typename U> struct B {
@@ -16,3 +15,76 @@ using T = A<void>::B<int>;
 
 using Copy = decltype(copy);
 using Copy = A<void>::B<int>;
+
+namespace GH94614 {
+
+template <class, class> struct S {};
+
+struct trouble_1 {
+} constexpr t1;
+struct trouble_2 {
+} constexpr t2;
+struct trouble_3 {
+} constexpr t3;
+struct trouble_4 {
+} constexpr t4;
+struct trouble_5 {
+} constexpr t5;
+struct trouble_6 {
+} constexpr t6;
+struct trouble_7 {
+} constexpr t7;
+struct trouble_8 {
+} constexpr t8;
+struct trouble_9 {
+} constexpr t9;
+
+template <class U, class... T> struct Unrelated {
+  using Trouble = S<U, T...>;
+
+  template <class... V> using Trouble2 = S<V..., T...>;
+};
+
+template <class T, class U> struct Outer {
+  using Trouble = S<U, T>;
+
+  template <class V> using Trouble2 = S<V, T>;
+
+  template <class V> using Trouble3 = S<U, T>;
+
+  template <class V> struct Inner {
+    template <class W> struct Paranoid {
+      using Trouble4 = S<W, T>;
+
+      template <class... X> using Trouble5 = S<X..., T>;
+    };
+
+    Inner(trouble_1, V v, Trouble trouble) {}
+    Inner(trouble_2, V v, Trouble2<V> trouble) {}
+    Inner(trouble_3, V v, Trouble3<V> trouble) {}
+    Inner(trouble_4, V v, typename Unrelated<U, T>::template Trouble2<V> trouble) {}
+    Inner(trouble_5, V v, typename Unrelated<U, T>::Trouble trouble) {}
+    Inner(trouble_6, V v, typename Unrelated<V, T>::Trouble trouble) {}
+    Inner(trouble_7, V v, typename Paranoid<V>::Trouble4 trouble) {}
+    Inner(trouble_8, V v, typename Paranoid<V>::template Trouble5<V> trouble) {}
+    template <class W>
+    Inner(trouble_9, V v, W w, typename Paranoid<V>::template Trouble5<W> trouble) {}
+  };
+};
+
+S<int, char> s;
+
+Outer<char, int>::Inner _1(t1, 42, s);
+Outer<char, int>::Inner _2(t2, 42, s);
+Outer<char, int>::Inner _3(t3, 42, s);
+Outer<char, int>::Inner _4(t4, 42, s);
+Outer<char, int>::Inner _5(t5, 42, s);
+Outer<char, int>::Inner _6(t6, 42, s);
+Outer<char, int>::Inner _7(t7, 42, s);
+Outer<char, int>::Inner _8(t8, 42, s);
+Outer<char, int>::Inner _9(t9, 42, 24, s);
+
+// Make sure we don't accidentally inject the TypedefNameDecl into the TU.
+Trouble should_not_be_in_the_tu_decl; // expected-error {{unknown type name 'Trouble'}}
+
+} // namespace GH94614