Index: clang/include/clang/Basic/Attr.td
===================================================================
--- clang/include/clang/Basic/Attr.td
+++ clang/include/clang/Basic/Attr.td
@@ -2848,6 +2848,7 @@
                  "IR_ARCInitReturnsUnrelated",
                  "IR_ARCFieldWithOwnership"], 1, /*fake*/ 1>];
   let Documentation = [Undocumented];
+  let MeaningfulToClassTemplateDefinition = 1;
 }
 
 def DiagnoseIf : InheritableAttr {
Index: clang/test/SemaCXX/attr-unavailable.cpp
===================================================================
--- clang/test/SemaCXX/attr-unavailable.cpp
+++ clang/test/SemaCXX/attr-unavailable.cpp
@@ -172,3 +172,13 @@
 
 template <class T>
 int phase_one_unavailable2(int x = unavailable_int()) __attribute__((unavailable)) {}
+
+namespace GH61815 {
+template <class _ValueType = int>
+class __attribute__((unavailable)) polymorphic_allocator {}; // expected-note 2 {{'polymorphic_allocator<void>' has been explicitly marked unavailable here}}
+
+void f() {
+  polymorphic_allocator<void> a; // expected-error {{'polymorphic_allocator<void>' is unavailable}}
+  polymorphic_allocator<void> b; // expected-error {{'polymorphic_allocator<void>' is unavailable}}
+}
+}