diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index f2b27893b7a9d..acd9dd9298ce1 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -886,6 +886,9 @@ Bug Fixes to C++ Support
   out of a module (which is the case e.g. in MSVC's implementation of ``std`` module). (#GH118218)
 - Fixed a pack expansion issue in checking unexpanded parameter sizes. (#GH17042)
 - Fixed a bug where captured structured bindings were modifiable inside non-mutable lambda (#GH95081)
+- Passing incomplete types to ``__is_base_of`` and other builtin type traits for which the corresponding
+  standard type trait mandates a complete type is now a hard (non-sfinae-friendly) error
+  (`LWG3929 <https://wg21.link/LWG3929>`__.) (#GH121278)
 - Clang now identifies unexpanded parameter packs within the type constraint on a non-type template parameter. (#GH88866)
 - Fixed an issue while resolving type of expression indexing into a pack of values of non-dependent type (#GH121242)
 
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 330ae045616ab..03fb7ca9bc3c3 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -9361,7 +9361,7 @@ def note_inequality_comparison_to_or_assign : Note<
   "use '|=' to turn this inequality comparison into an or-assignment">;
 
 def err_incomplete_type_used_in_type_trait_expr : Error<
-  "incomplete type %0 used in type trait expression">;
+  "incomplete type %0 used in type trait expression">, NoSFINAE;
 
 // C++20 constinit and require_constant_initialization attribute
 def warn_cxx20_compat_constinit : Warning<
diff --git a/clang/test/SemaCXX/type-traits.cpp b/clang/test/SemaCXX/type-traits.cpp
index 91ef7786f11bb..1b9e2ba6ff162 100644
--- a/clang/test/SemaCXX/type-traits.cpp
+++ b/clang/test/SemaCXX/type-traits.cpp
@@ -5031,3 +5031,18 @@ void remove_all_extents() {
   using SomeArray = int[1][2];
   static_assert(__is_same(remove_all_extents_t<const SomeArray>, const int));
 }
+
+namespace GH121278 {
+// https://cplusplus.github.io/LWG/lwg-active.html#3929
+#if __cplusplus >= 202002L
+template <typename B, typename D>
+concept C = __is_base_of(B, D);
+// expected-error@-1 {{incomplete type 'GH121278::S' used in type trait expression}}
+// expected-note@-2 {{while substituting template arguments into constraint expression here}}
+
+struct T;
+struct S;
+bool b = C<T, S>;
+// expected-note@-1 {{while checking the satisfaction of concept 'C<GH121278::T, GH121278::S>' requested here}}
+#endif
+}