[Clang][Attributes] Add MeaningfulToClassTemplateDefinition to unavailable attribute

There may be cases in which we want to diagnose a type as unavailable but it may
not be complete at the time. Setting MeaningfulToClassTemplateDefinition fixes
this issue.

This fixes: https://github.com/llvm/llvm-project/issues/61815

Differential Revision: https://reviews.llvm.org/D147495

clang/docs/ReleaseNotes.rst

@@ -210,6 +210,9 @@ Improvements to Clang's diagnostics
 - Clang now checks for completeness of the second and third arguments in the
   conditional operator.
   (`#59718 <https://github.com/llvm/llvm-project/issues/59718>`_)
+- There were some cases in which the diagnostic for the unavailable attribute
+  might not be issued, this fixes those cases.
+  (`61815 <https://github.com/llvm/llvm-project/issues/61815>`_)
 
 Bug Fixes in This Version
 -------------------------

clang/include/clang/Basic/Attr.td

@@ -2848,6 +2848,7 @@ def Unavailable : InheritableAttr {
                  "IR_ARCInitReturnsUnrelated",
                  "IR_ARCFieldWithOwnership"], 1, /*fake*/ 1>];
   let Documentation = [Undocumented];
+  let MeaningfulToClassTemplateDefinition = 1;
 }
 
 def DiagnoseIf : InheritableAttr {

clang/test/SemaCXX/attr-unavailable.cpp

@@ -172,3 +172,13 @@ int phase_one_unavailable(int x = unavailable_int()) {}
 
 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}}
+}
+}