[clang] Substitute alias templates from correct context (#75069)

Current context set to where alias was met, not where it is declared
caused incorrect access check in case alias referenced private members
of the parent class.
This is a recommit of 6b1aa31 with a slight modification in order to fix
reported regression.

Fixes #41693

clang/docs/ReleaseNotes.rst

@@ -685,6 +685,9 @@ Bug Fixes in This Version
   (`#62157 <https://github.com/llvm/llvm-project/issues/62157>`_) and
   (`#64885 <https://github.com/llvm/llvm-project/issues/64885>`_) and
   (`#65568 <https://github.com/llvm/llvm-project/issues/65568>`_)
+- Fixed false positive error emitted when templated alias inside a class
+  used private members of the same class.
+  Fixes (`#41693 <https://github.com/llvm/llvm-project/issues/41693>`_)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Sema/Sema.h

@@ -8737,7 +8737,7 @@ class Sema final {
                              SourceLocation IILoc,
                              bool DeducedTSTContext = true);
 
-
+  bool RebuildingTypesInCurrentInstantiation = false;
   TypeSourceInfo *RebuildTypeInCurrentInstantiation(TypeSourceInfo *T,
                                                     SourceLocation Loc,
                                                     DeclarationName Name);

clang/lib/Sema/SemaCXXScopeSpec.cpp

@@ -30,17 +30,33 @@ static CXXRecordDecl *getCurrentInstantiationOf(QualType T,
     return nullptr;
 
   const Type *Ty = T->getCanonicalTypeInternal().getTypePtr();
+  if (isa<TemplateSpecializationType>(Ty)) {
+    if (auto *Record = dyn_cast<CXXRecordDecl>(CurContext)) {
+      if (isa<ClassTemplatePartialSpecializationDecl>(Record) ||
+          Record->getDescribedClassTemplate()) {
+        const Type *ICNT = Record->getTypeForDecl();
+        QualType Injected =
+            cast<InjectedClassNameType>(ICNT)->getInjectedSpecializationType();
+
+        if (Ty == Injected->getCanonicalTypeInternal().getTypePtr())
+          return Record;
+      }
+    }
+  }
+
   if (const RecordType *RecordTy = dyn_cast<RecordType>(Ty)) {
     CXXRecordDecl *Record = cast<CXXRecordDecl>(RecordTy->getDecl());
     if (!Record->isDependentContext() ||
         Record->isCurrentInstantiation(CurContext))
       return Record;
 
     return nullptr;
-  } else if (isa<InjectedClassNameType>(Ty))
-    return cast<InjectedClassNameType>(Ty)->getDecl();
-  else
-    return nullptr;
+  }
+
+  if (auto *ICNT = dyn_cast<InjectedClassNameType>(Ty))
+    return ICNT->getDecl();
+
+  return nullptr;
 }
 
 /// Compute the DeclContext that is associated with the given type.

clang/lib/Sema/SemaTemplate.cpp

@@ -39,6 +39,7 @@
 #include "llvm/ADT/SmallBitVector.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringExtras.h"
+#include "llvm/Support/SaveAndRestore.h"
 
 #include <iterator>
 #include <optional>
@@ -3990,9 +3991,14 @@ QualType Sema::CheckTemplateIdType(TemplateName Name,
     if (Inst.isInvalid())
       return QualType();
 
-    CanonType = SubstType(Pattern->getUnderlyingType(),
-                          TemplateArgLists, AliasTemplate->getLocation(),
-                          AliasTemplate->getDeclName());
+    {
+      Sema::ContextRAII SavedContext(*this, Pattern->getDeclContext());
+      if (RebuildingTypesInCurrentInstantiation)
+        SavedContext.pop();
+      CanonType =
+          SubstType(Pattern->getUnderlyingType(), TemplateArgLists,
+                    AliasTemplate->getLocation(), AliasTemplate->getDeclName());
+    }
     if (CanonType.isNull()) {
       // If this was enable_if and we failed to find the nested type
       // within enable_if in a SFINAE context, dig out the specific
@@ -11392,6 +11398,8 @@ TypeSourceInfo *Sema::RebuildTypeInCurrentInstantiation(TypeSourceInfo *T,
   if (!T || !T->getType()->isInstantiationDependentType())
     return T;
 
+  llvm::SaveAndRestore DisableContextSwitchForTypeAliases(
+      RebuildingTypesInCurrentInstantiation, true);
   CurrentInstantiationRebuilder Rebuilder(*this, Loc, Name);
   return Rebuilder.TransformType(T);
 }

clang/test/CXX/temp/temp.decls/temp.alias/p3.cpp

@@ -2,11 +2,12 @@
 
 // The example given in the standard (this is rejected for other reasons anyway).
 template<class T> struct A;
-template<class T> using B = typename A<T>::U; // expected-error {{no type named 'U' in 'A<T>'}}
+template<class T> using B = typename A<T>::U; // expected-error {{no type named 'U' in 'A<short>'}}
+                                              // expected-note@-1 {{in instantiation of template class 'A<short>' requested here}}
 template<class T> struct A {
   typedef B<T> U; // expected-note {{in instantiation of template type alias 'B' requested here}}
 };
-B<short> b;
+B<short> b; // expected-note {{in instantiation of template type alias 'B' requested here}}
 
 template<typename T> using U = int;
 

clang/test/SemaCXX/alias-template.cpp

@@ -192,3 +192,68 @@ int g = sfinae_me<int>(); // expected-error{{no matching function for call to 's
 namespace NullExceptionDecl {
 template<int... I> auto get = []() { try { } catch(...) {}; return I; }; // expected-error{{initializer contains unexpanded parameter pack 'I'}}
 }
+
+namespace GH41693 {
+// No errors when a type alias defined in a class or a friend of a class
+// accesses private members of the same class.
+struct S {
+private:
+  template <typename> static constexpr void Impl() {}
+
+public:
+  template <typename X> using U = decltype(Impl<X>());
+};
+
+using X = S::U<void>;
+struct Y {
+private:
+  static constexpr int x=0;
+
+  template <typename>
+  static constexpr int y=0;
+
+  template <typename>
+  static constexpr int foo();
+
+public:
+  template <typename U>
+  using bar1 = decltype(foo<U>());
+  using bar2 = decltype(x);
+  template <typename U>
+  using bar3 = decltype(y<U>);
+};
+
+
+using type1 = Y::bar1<float>;
+using type2 = Y::bar2;
+using type3 = Y::bar3<float>;
+
+struct theFriend{
+  template<class T>
+  using theAlias = decltype(&T::i);
+};
+
+class theC{
+  int i;
+  public:
+  friend struct theFriend;
+};
+
+int foo(){
+  (void)sizeof(theFriend::theAlias<theC>);
+}
+
+// Test case that regressed with the first iteration of the fix for GH41693.
+template <typename T> class SP {
+    T* data;
+};
+
+template <typename T> class A {
+    static SP<A> foo();
+};
+
+template<typename T> using TRet = SP<A<T>>;
+
+template<typename T> TRet<T> A<T>::foo() { return TRet<T>{};};
+
+}