[clang][Sema] Fix a CTAD regression after 42239d2 (#86914)

The most recent declaration of a template as a friend can introduce a
different template parameter depth compared to what we anticipate from a
CTAD guide.

Fixes #86769

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -1106,6 +1106,10 @@ Bug Fixes to C++ Support
 - Fix a crash when an unresolved overload set is encountered on the RHS of a ``.*`` operator.
   (`#53815 <https://github.com/llvm/llvm-project/issues/53815>`_)
 
+- Fixed a regression in CTAD that a friend declaration that befriends itself may cause
+  incorrect constraint substitution.
+  (`#86769 <https://github.com/llvm/llvm-project/issues/86769>`_)
+
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 - Fixed an import failure of recursive friend class template.

clang/lib/Sema/SemaTemplate.cpp

@@ -1830,7 +1830,27 @@ static TemplateParameterList *GetTemplateParameterList(TemplateDecl *TD) {
   // Make sure we get the template parameter list from the most
   // recent declaration, since that is the only one that is guaranteed to
   // have all the default template argument information.
-  return cast<TemplateDecl>(TD->getMostRecentDecl())->getTemplateParameters();
+  Decl *D = TD->getMostRecentDecl();
+  // C++11 [temp.param]p12:
+  // A default template argument shall not be specified in a friend class
+  // template declaration.
+  //
+  // Skip past friend *declarations* because they are not supposed to contain
+  // default template arguments. Moreover, these declarations may introduce
+  // template parameters living in different template depths than the
+  // corresponding template parameters in TD, causing unmatched constraint
+  // substitution.
+  //
+  // FIXME: Diagnose such cases within a class template:
+  //  template <class T>
+  //  struct S {
+  //    template <class = void> friend struct C;
+  //  };
+  //  template struct S<int>;
+  while (D->getFriendObjectKind() != Decl::FriendObjectKind::FOK_None &&
+         D->getPreviousDecl())
+    D = D->getPreviousDecl();
+  return cast<TemplateDecl>(D)->getTemplateParameters();
 }
 
 DeclResult Sema::CheckClassTemplate(

clang/test/SemaTemplate/concepts-friends.cpp

@@ -478,3 +478,29 @@ template <Concept> class Foo {
 };
 
 } // namespace FriendOfFriend
+
+namespace GH86769 {
+
+template <typename T>
+concept X = true;
+
+template <X T> struct Y {
+  Y(T) {}
+  template <X U> friend struct Y;
+  template <X U> friend struct Y;
+  template <X U> friend struct Y;
+};
+
+template <class T>
+struct Z {
+  // FIXME: This is ill-formed per C++11 [temp.param]p12:
+  // A default template argument shall not be specified in a friend class
+  // template declaration.
+  template <X U = void> friend struct Y;
+};
+
+template struct Y<int>;
+template struct Z<int>;
+Y y(1);
+
+}

clang/test/SemaTemplate/ctad.cpp

@@ -53,4 +53,4 @@ X x;
 template<class T, class B> struct Y { Y(T); };
 template<class T, class B=void> struct Y ;
 Y y(1);
-};
+}