diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -3519,6 +3519,7 @@
     }
 
     case tok::kw___is_signed:
+    case tok::kw___is_unsigned:
       // GNU libstdc++ 4.4 uses __is_signed as an identifier, but Clang
       // typically treats it as a trait. If we see __is_signed as it appears
       // in libstdc++, e.g.,
@@ -3526,8 +3527,15 @@
       //   static const bool __is_signed;
       //
       // then treat __is_signed as an identifier rather than as a keyword.
+      //
+      // Similarly, libstdc++ 13 trunk at one point used __is_unsigned as an
+      // identifier as
+      //
+      //   static constexpr bool __is_unsigned;
+      //
       if (DS.getTypeSpecType() == TST_bool &&
-          DS.getTypeQualifiers() == DeclSpec::TQ_const &&
+          (DS.getTypeQualifiers() == DeclSpec::TQ_const ||
+           DS.getConstexprSpecifier() == ConstexprSpecKind::Constexpr) &&
           DS.getStorageClassSpec() == DeclSpec::SCS_static)
         TryKeywordIdentFallback(true);
 
diff --git a/clang/test/SemaCXX/libstdcxx_is_pod_hack.cpp b/clang/test/SemaCXX/libstdcxx_is_pod_hack.cpp
--- a/clang/test/SemaCXX/libstdcxx_is_pod_hack.cpp
+++ b/clang/test/SemaCXX/libstdcxx_is_pod_hack.cpp
@@ -29,6 +29,13 @@
 
 bool check_signed = test_is_signed::__is_signed;
 
+// Another, similar egregious hack for __is_unsigned, which is a type
+// trait in Embarcadero's compiler but is used as an identifier in
+// libstdc++.
+struct test_is_unsigned {
+  static constexpr bool __is_unsigned = true; // expected-warning {{keyword '__is_unsigned' will be made available as an identifier}}
+};
+
 template<bool B> struct must_be_true {};
 template<> struct must_be_true<false>;