[Clang][Sema] Diagnose unexpanded packs in the template argument lists of function template specializations (#76677)

This diagnoses unexpanded packs in the _unqualified-id_ of a function
template specialization's _declarator-id_. For example:
```cpp
template<typename... Ts>
struct A
{
    template<typename U>
    void f();

    template<>
    void f<Ts>(); // error: explicit specialization contains unexpanded parameter pack 'Ts'
};
```

I moved the handling of template-id's so it happens right after we
determine whether we are declaring a function template/function template
specialization so diagnostics are issued in lexical order.

Author: sdkrystian

clang/docs/ReleaseNotes.rst

@@ -518,6 +518,7 @@ Improvements to Clang's diagnostics
 - Clang now diagnoses definitions of friend function specializations, e.g. ``friend void f<>(int) {}``.
 - Clang now diagnoses narrowing conversions involving const references.
   (`#63151: <https://github.com/llvm/llvm-project/issues/63151>`_).
+- Clang now diagnoses unexpanded packs within the template argument lists of function template specializations.
 
 
 Improvements to Clang's time-trace

clang/lib/Sema/SemaDecl.cpp

@@ -9900,15 +9900,15 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
     // Match up the template parameter lists with the scope specifier, then
     // determine whether we have a template or a template specialization.
     bool Invalid = false;
+    TemplateIdAnnotation *TemplateId =
+        D.getName().getKind() == UnqualifiedIdKind::IK_TemplateId
+            ? D.getName().TemplateId
+            : nullptr;
     TemplateParameterList *TemplateParams =
         MatchTemplateParametersToScopeSpecifier(
             D.getDeclSpec().getBeginLoc(), D.getIdentifierLoc(),
-            D.getCXXScopeSpec(),
-            D.getName().getKind() == UnqualifiedIdKind::IK_TemplateId
-                ? D.getName().TemplateId
-                : nullptr,
-            TemplateParamLists, isFriend, isMemberSpecialization,
-            Invalid);
+            D.getCXXScopeSpec(), TemplateId, TemplateParamLists, isFriend,
+            isMemberSpecialization, Invalid);
     if (TemplateParams) {
       // Check that we can declare a template here.
       if (CheckTemplateDeclScope(S, TemplateParams))
@@ -9921,6 +9921,11 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
         if (Name.getNameKind() == DeclarationName::CXXDestructorName) {
           Diag(NewFD->getLocation(), diag::err_destructor_template);
           NewFD->setInvalidDecl();
+          // Function template with explicit template arguments.
+        } else if (TemplateId) {
+          Diag(D.getIdentifierLoc(), diag::err_function_template_partial_spec)
+              << SourceRange(TemplateId->LAngleLoc, TemplateId->RAngleLoc);
+          NewFD->setInvalidDecl();
         }
 
         // If we're adding a template to a dependent context, we may need to
@@ -9973,6 +9978,11 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
             << FixItHint::CreateRemoval(RemoveRange)
             << FixItHint::CreateInsertion(InsertLoc, "<>");
           Invalid = true;
+
+          // Recover by faking up an empty template argument list.
+          HasExplicitTemplateArgs = true;
+          TemplateArgs.setLAngleLoc(InsertLoc);
+          TemplateArgs.setRAngleLoc(InsertLoc);
         }
       }
     } else {
@@ -9986,6 +9996,33 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
       if (TemplateParamLists.size() > 0)
         // For source fidelity, store all the template param lists.
         NewFD->setTemplateParameterListsInfo(Context, TemplateParamLists);
+
+      // "friend void foo<>(int);" is an implicit specialization decl.
+      if (isFriend && TemplateId)
+        isFunctionTemplateSpecialization = true;
+    }
+
+    // If this is a function template specialization and the unqualified-id of
+    // the declarator-id is a template-id, convert the template argument list
+    // into our AST format and check for unexpanded packs.
+    if (isFunctionTemplateSpecialization && TemplateId) {
+      HasExplicitTemplateArgs = true;
+
+      TemplateArgs.setLAngleLoc(TemplateId->LAngleLoc);
+      TemplateArgs.setRAngleLoc(TemplateId->RAngleLoc);
+      ASTTemplateArgsPtr TemplateArgsPtr(TemplateId->getTemplateArgs(),
+                                         TemplateId->NumArgs);
+      translateTemplateArguments(TemplateArgsPtr, TemplateArgs);
+
+      // FIXME: Should we check for unexpanded packs if this was an (invalid)
+      // declaration of a function template partial specialization? Should we
+      // consider the unexpanded pack context to be a partial specialization?
+      for (const TemplateArgumentLoc &ArgLoc : TemplateArgs.arguments()) {
+        if (DiagnoseUnexpandedParameterPack(
+                ArgLoc, isFriend ? UPPC_FriendDeclaration
+                                 : UPPC_ExplicitSpecialization))
+          NewFD->setInvalidDecl();
+      }
     }
 
     if (Invalid) {
@@ -10438,46 +10475,6 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
            diag::ext_operator_new_delete_declared_inline)
         << NewFD->getDeclName();
 
-    // If the declarator is a template-id, translate the parser's template
-    // argument list into our AST format.
-    if (D.getName().getKind() == UnqualifiedIdKind::IK_TemplateId) {
-      TemplateIdAnnotation *TemplateId = D.getName().TemplateId;
-      TemplateArgs.setLAngleLoc(TemplateId->LAngleLoc);
-      TemplateArgs.setRAngleLoc(TemplateId->RAngleLoc);
-      ASTTemplateArgsPtr TemplateArgsPtr(TemplateId->getTemplateArgs(),
-                                         TemplateId->NumArgs);
-      translateTemplateArguments(TemplateArgsPtr,
-                                 TemplateArgs);
-
-      HasExplicitTemplateArgs = true;
-
-      if (NewFD->isInvalidDecl()) {
-        HasExplicitTemplateArgs = false;
-      } else if (FunctionTemplate) {
-        // Function template with explicit template arguments.
-        Diag(D.getIdentifierLoc(), diag::err_function_template_partial_spec)
-          << SourceRange(TemplateId->LAngleLoc, TemplateId->RAngleLoc);
-
-        HasExplicitTemplateArgs = false;
-      } else if (isFriend) {
-        // "friend void foo<>(int);" is an implicit specialization decl.
-        isFunctionTemplateSpecialization = true;
-      } else {
-        assert(isFunctionTemplateSpecialization &&
-               "should have a 'template<>' for this decl");
-      }
-    } else if (isFriend && isFunctionTemplateSpecialization) {
-      // This combination is only possible in a recovery case;  the user
-      // wrote something like:
-      //   template <> friend void foo(int);
-      // which we're recovering from as if the user had written:
-      //   friend void foo<>(int);
-      // Go ahead and fake up a template id.
-      HasExplicitTemplateArgs = true;
-      TemplateArgs.setLAngleLoc(D.getIdentifierLoc());
-      TemplateArgs.setRAngleLoc(D.getIdentifierLoc());
-    }
-
     // We do not add HD attributes to specializations here because
     // they may have different constexpr-ness compared to their
     // templates and, after maybeAddCUDAHostDeviceAttrs() is applied,

clang/test/CXX/temp/temp.decls/temp.variadic/p5.cpp

@@ -376,6 +376,11 @@ namespace Specializations {
   template<typename... Ts>
   struct PrimaryClass<Ts>; // expected-error{{partial specialization contains unexpanded parameter pack 'Ts'}}
 
+  template<typename T, typename... Ts>
+  void PrimaryFunction();
+  template<typename T, typename... Ts>
+  void PrimaryFunction<Ts>(); // expected-error{{function template partial specialization is not allowed}}
+
 #if __cplusplus >= 201402L
   template<typename T, typename... Ts>
   constexpr int PrimaryVar = 0;
@@ -392,6 +397,13 @@ namespace Specializations {
     template<typename U>
     struct InnerClass<U, Ts>; // expected-error{{partial specialization contains unexpanded parameter pack 'Ts'}}
 
+    template<typename... Us>
+    void InnerFunction();
+    template<>
+    void InnerFunction<Ts>(); // expected-error{{explicit specialization contains unexpanded parameter pack 'Ts'}}
+
+    friend void PrimaryFunction<Ts>(); // expected-error{{friend declaration contains unexpanded parameter pack 'Ts'}}
+
 #if __cplusplus >= 201402L
     template<typename... Us>
     constexpr static int InnerVar = 0;