[clang] [clang][Sema] Fix a CTAD regression after 42239d2e9 (PR #86914)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Younan Zhang (zyn0217)

<details>
<summary>Changes</summary>

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 https://github.com/llvm/llvm-project/issues/86769

---
Full diff: https://github.com/llvm/llvm-project/pull/86914.diff


4 Files Affected:

- (modified) clang/docs/ReleaseNotes.rst (+3) 
- (modified) clang/lib/Sema/SemaTemplate.cpp (+13-1) 
- (modified) clang/test/SemaTemplate/concepts-friends.cpp (+24) 
- (modified) clang/test/SemaTemplate/ctad.cpp (+1-1) 


``````````diff
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 7fbe2fec6ca065..ed0996bd3d30c0 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/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
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 005529a53270c3..14fabe36eee794 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/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(
diff --git a/clang/test/SemaTemplate/concepts-friends.cpp b/clang/test/SemaTemplate/concepts-friends.cpp
index 255b0858917fb6..31840309f1ec86 100644
--- a/clang/test/SemaTemplate/concepts-friends.cpp
+++ b/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);
+
+}
diff --git a/clang/test/SemaTemplate/ctad.cpp b/clang/test/SemaTemplate/ctad.cpp
index 388ed7d4cced18..ec144d4f44ba8c 100644
--- a/clang/test/SemaTemplate/ctad.cpp
+++ b/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);
-};
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/86914