2 files changed, 42 insertions, 0 deletions
diff --git a/absl/debugging/internal/demangle.cc b/absl/debugging/internal/demangle.cc
index 7901a410..42c10bbe 100644
--- a/absl/debugging/internal/demangle.cc
+++ b/absl/debugging/internal/demangle.cc
@@ -823,8 +823,13 @@ static bool ParsePrefix(State *state) {
 //                    ::= <local-source-name> [<abi-tags>]
 //                    ::= <unnamed-type-name> [<abi-tags>]
 //                    ::= DC <source-name>+ E  # C++17 structured binding
+//                    ::= F <source-name>  # C++20 constrained friend
+//                    ::= F <operator-name>  # C++20 constrained friend
 //
 // <local-source-name> is a GCC extension; see below.
+//
+// For the F notation for constrained friends, see
+// https://github.com/itanium-cxx-abi/cxx-abi/issues/24#issuecomment-1491130332.
 static bool ParseUnqualifiedName(State *state) {
   ComplexityGuard guard(state);
   if (guard.IsTooComplex()) return false;
@@ -841,6 +846,15 @@ static bool ParseUnqualifiedName(State *state) {
     return true;
   }
   state->parse_state = copy;
+
+  // F <source-name>
+  // F <operator-name>
+  if (ParseOneCharToken(state, 'F') && MaybeAppend(state, "friend ") &&
+      (ParseSourceName(state) || ParseOperatorName(state, nullptr))) {
+    return true;
+  }
+  state->parse_state = copy;
+
   return false;
 }
 
diff --git a/absl/debugging/internal/demangle_test.cc b/absl/debugging/internal/demangle_test.cc
index 1d108c8b..68ab8a3c 100644
--- a/absl/debugging/internal/demangle_test.cc
+++ b/absl/debugging/internal/demangle_test.cc
@@ -160,6 +160,34 @@ TEST(Demangle, ConstrainedAutoInFunctionTemplate) {
   EXPECT_STREQ(tmp, "f<>()");
 }
 
+TEST(Demangle, ConstrainedFriendFunctionTemplate) {
+  char tmp[100];
+
+  // Source:
+  //
+  // namespace ns {
+  // template <class T> struct Y {
+  //   friend void y(Y) requires true {}
+  // };
+  // }  // namespace ns
+  //
+  // y(ns::Y<int>{});
+  //
+  // LLVM demangling:
+  //
+  // ns::Y<int>::friend y(ns::Y<int>) requires true
+  ASSERT_TRUE(Demangle("_ZN2ns1YIiEF1yES1_QLb1E", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "ns::Y<>::friend y()");
+}
+
+TEST(Demangle, ConstrainedFriendOperatorTemplate) {
+  char tmp[100];
+
+  // ns::Y<int>::friend operator*(ns::Y<int>) requires true
+  ASSERT_TRUE(Demangle("_ZN2ns1YIiEFdeES1_QLb1E", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "ns::Y<>::friend operator*()");
+}
+
 TEST(Demangle, NonTemplateBuiltinType) {
   char tmp[100];