[clang] [Clang] fix generic lambda inside requires-clause of friend function template (PR #99813)

[Zhikai Zeng via cfe-commits]

https://github.com/Backl1ght created https://github.com/llvm/llvm-project/pull/99813

fixes https://github.com/llvm/llvm-project/issues/98258

The cause is that the assertion "Nothing should reference a value below the actual template depth" is incorrect since we can have a generic lambda inside requires-clause of friend function template, and the generic lambda can reference to values with greater template depth.

>From 4087cdd60a86a239dfea62344bb30e954d5cc662 Mon Sep 17 00:00:00 2001
From: Backl1ght <backlight.zzk at gmail.com>
Date: Sun, 21 Jul 2024 23:43:24 +0800
Subject: [PATCH] fix

---
 clang/docs/ReleaseNotes.rst                  |  2 ++
 clang/lib/Sema/SemaTemplate.cpp              |  5 +----
 clang/test/SemaTemplate/concepts-friends.cpp | 16 ++++++++++++++++
 3 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 4638b91b48f95..50da6d4556133 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -1083,6 +1083,8 @@ Bug Fixes to C++ Support
 - Clang now diagnoses explicit object parameters in member pointers and other contexts where they should not appear.
   Fixes (#GH85992).
 - Fixed a crash-on-invalid bug involving extraneous template parameter with concept substitution. (#GH73885)
+- Fixed an assertion failure about a constraint of a friend function template references to a value with greater
+  template depth than the friend function template. (#GH98258)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 9d96201625389..c2dd5b04f1f70 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -1691,10 +1691,7 @@ class ConstraintRefersToContainingTemplateChecker
   using inherited::TransformTemplateTypeParmType;
   QualType TransformTemplateTypeParmType(TypeLocBuilder &TLB,
                                          TemplateTypeParmTypeLoc TL, bool) {
-    assert(TL.getDecl()->getDepth() <= TemplateDepth &&
-           "Nothing should reference a value below the actual template depth, "
-           "depth is likely wrong");
-    if (TL.getDecl()->getDepth() != TemplateDepth)
+    if (TL.getDecl()->getDepth() < TemplateDepth)
       Result = true;
     return inherited::TransformTemplateTypeParmType(
         TLB, TL,
diff --git a/clang/test/SemaTemplate/concepts-friends.cpp b/clang/test/SemaTemplate/concepts-friends.cpp
index 91b797034ed6c..9a77aad24f1cd 100644
--- a/clang/test/SemaTemplate/concepts-friends.cpp
+++ b/clang/test/SemaTemplate/concepts-friends.cpp
@@ -504,3 +504,19 @@ template struct Z<int>;
 Y y(1);
 
 }
+
+namespace GH98258 {
+
+struct S {
+  template <typename U>
+  friend void f() requires requires { []<typename V>(V){}; } {
+    return;
+  }
+
+  template <typename U>
+  friend void f2() requires requires { [](auto){}; } {
+    return;
+  }
+};
+
+}