[clang][Sema] Fix a CTAD regression after 42239d2

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 llvm#86769

clang/docs/ReleaseNotes.rst

@@ -345,6 +345,9 @@ Bug Fixes in This Version
 - Fixes an assertion failure on invalid code when trying to define member
   functions in lambdas.
 
+- Fixed a regression in CTAD that a friend declaration that befriends itself may cause
+  incorrect constraint substitution. (#GH86769).
+
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

clang/lib/Sema/SemaTemplate.cpp

@@ -1836,7 +1836,19 @@ 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 *ND = TD->getMostRecentDecl();
+  // Skip past friend Decls 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.
+  //
+  // C++23 N4950 [temp.param]p12
+  // A default template argument shall not be specified in a friend class
+  // template declaration.
+  while (ND->getFriendObjectKind() != Decl::FriendObjectKind::FOK_None &&
+         ND->getPreviousDecl())
+    ND = ND->getPreviousDecl();
+  return cast<TemplateDecl>(ND)->getTemplateParameters();
 }
 
 DeclResult Sema::CheckClassTemplate(

clang/test/SemaTemplate/concepts-friends.cpp

@@ -478,3 +478,27 @@ 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 N4950 [temp.param]p12.
+  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);
-};
+}