[clang] Diagnose use of deprecated template alias (#97619)

Issue a warning diagnostic when a template alias with a deprecated
attribute is used.

clang/docs/ReleaseNotes.rst

@@ -745,6 +745,19 @@ Improvements to Clang's diagnostics
 - Clang now diagnoses dangling assignments for pointer-like objects (annotated with `[[gsl::Pointer]]`) under `-Wdangling-assignment-gsl` (off by default)
   Fixes #GH63310.
   
+- Clang now diagnoses uses of alias templates with a deprecated attribute. (Fixes #GH18236).
+
+  .. code-block:: c++
+
+     template <typename T>
+     struct NoAttr {
+     };
+
+     template <typename T>
+     using UsingWithAttr __attribute__((deprecated)) = NoAttr<T>;
+
+     UsingWithAttr<int> objUsingWA; // warning: 'UsingWithAttr' is deprecated
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/lib/Sema/SemaAvailability.cpp

@@ -107,6 +107,12 @@ ShouldDiagnoseAvailabilityOfDecl(Sema &S, const NamedDecl *D,
     break;
   }
 
+  // For alias templates, get the underlying declaration.
+  if (const auto *ADecl = dyn_cast<TypeAliasTemplateDecl>(D)) {
+    D = ADecl->getTemplatedDecl();
+    Result = D->getAvailability(Message);
+  }
+
   // Forward class declarations get their attributes from their definition.
   if (const auto *IDecl = dyn_cast<ObjCInterfaceDecl>(D)) {
     if (IDecl->getDefinition()) {

clang/lib/Sema/SemaTemplate.cpp

@@ -3344,6 +3344,10 @@ QualType Sema::CheckTemplateIdType(TemplateName Name,
         *this, /*PointOfInstantiation=*/TemplateLoc,
         /*Entity=*/AliasTemplate,
         /*TemplateArgs=*/TemplateArgLists.getInnermost());
+
+    // Diagnose uses of this alias.
+    (void)DiagnoseUseOfDecl(AliasTemplate, TemplateLoc);
+
     if (Inst.isInvalid())
       return QualType();
 

clang/test/SemaTemplate/alias-template-deprecated.cpp

@@ -0,0 +1,83 @@
+// RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify %s
+//
+// This test checks that a deprecated attribute on an alias
+// template triggers a warning diagnostic when it is used.
+
+template <typename T>
+struct NoAttr {
+  void foo() {}
+};
+
+// expected-note@+2 7{{'UsingWithAttr' has been explicitly marked deprecated here}}
+template <typename T>
+using UsingWithAttr __attribute__((deprecated)) = NoAttr<T>;
+
+// expected-note@+1 {{'UsingInstWithAttr' has been explicitly marked deprecated here}}
+using UsingInstWithAttr __attribute__((deprecated)) = NoAttr<int>;
+
+// expected-note@+1 {{'TDWithAttr' has been explicitly marked deprecated here}}
+typedef NoAttr<int> TDWithAttr __attribute__((deprecated));
+
+// expected-warning@+1 {{'UsingWithAttr' is deprecated}}
+typedef UsingWithAttr<int> TDUsingWithAttr;
+
+typedef NoAttr<int> TDNoAttr;
+
+// expected-note@+1 {{'UsingTDWithAttr' has been explicitly marked deprecated here}}
+using UsingTDWithAttr __attribute__((deprecated)) = TDNoAttr;
+
+struct S {
+  NoAttr<float> f1;
+  // expected-warning@+1 {{'UsingWithAttr' is deprecated}}
+  UsingWithAttr<float> f2;
+};
+
+// expected-warning@+1 {{'UsingWithAttr' is deprecated}}
+void foo(NoAttr<short> s1, UsingWithAttr<short> s2) {
+}
+
+// expected-note@+2 {{'UsingWithCPPAttr' has been explicitly marked deprecated here}}
+template <typename T>
+using UsingWithCPPAttr [[deprecated]] = NoAttr<T>;
+
+// expected-note@+1 {{'UsingInstWithCPPAttr' has been explicitly marked deprecated here}}
+using UsingInstWithCPPAttr [[deprecated("Do not use this")]] = NoAttr<int>;
+
+void bar() {
+  NoAttr<int> obj; // Okay
+
+  // expected-warning@+2 {{'UsingWithAttr' is deprecated}}
+  // expected-note@+1 {{in instantiation of template type alias 'UsingWithAttr' requested here}}
+  UsingWithAttr<int> objUsingWA;
+
+  // expected-warning@+2 {{'UsingWithAttr' is deprecated}}
+  // expected-note@+1 {{in instantiation of template type alias 'UsingWithAttr' requested here}}
+  NoAttr<UsingWithAttr<int>> s;
+
+  // expected-note@+1 {{'DepInt' has been explicitly marked deprecated here}}
+  using DepInt [[deprecated]] = int;
+  // expected-warning@+3 {{'UsingWithAttr' is deprecated}}
+  // expected-warning@+2 {{'DepInt' is deprecated}}
+  // expected-note@+1 {{in instantiation of template type alias 'UsingWithAttr' requested here}}
+  using X = UsingWithAttr<DepInt>;
+
+  // expected-warning@+2 {{'UsingWithAttr' is deprecated}}
+  // expected-note@+1 {{in instantiation of template type alias 'UsingWithAttr' requested here}}
+  UsingWithAttr<int>().foo();
+
+  // expected-warning@+1 {{'UsingInstWithAttr' is deprecated}}
+  UsingInstWithAttr objUIWA;
+
+  // expected-warning@+1 {{'TDWithAttr' is deprecated}}
+  TDWithAttr objTDWA;
+
+  // expected-warning@+1 {{'UsingTDWithAttr' is deprecated}}
+  UsingTDWithAttr objUTDWA;
+
+  // expected-warning@+2 {{'UsingWithCPPAttr' is deprecated}}
+  // expected-note@+1 {{in instantiation of template type alias 'UsingWithCPPAttr' requested here}}
+  UsingWithCPPAttr<int> objUsingWCPPA;
+
+  // expected-warning@+1 {{'UsingInstWithCPPAttr' is deprecated: Do not use this}}
+  UsingInstWithCPPAttr objUICPPWA;
+}