[Sema] Substitute parameter packs when deduced from function arguments

clang/docs/ReleaseNotes.rst

@@ -70,6 +70,9 @@ C++2c Feature Support
 
 Resolutions to C++ Defect Reports
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+- Substitute template parameter pack, when it is not explicitly specified
+  in the template parameters, but is deduced from a previous argument.
+  (`#78449: <https://github.com/llvm/llvm-project/issues/78449>`_).
 
 C Language Changes
 ------------------

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -730,6 +730,7 @@ class PackDeductionScope {
   void addPack(unsigned Index) {
     // Save the deduced template argument for the parameter pack expanded
     // by this pack expansion, then clear out the deduction.
+    DeducedFromEarlierParameter = !Deduced[Index].isNull();
     DeducedPack Pack(Index);
     Pack.Saved = Deduced[Index];
     Deduced[Index] = TemplateArgument();
@@ -858,6 +859,27 @@ class PackDeductionScope {
       Info.PendingDeducedPacks[Pack.Index] = Pack.Outer;
   }
 
+  // Return the size of the saved packs if all of them has the same size.
+  std::optional<unsigned> getSavedPackSizeIfAllEqual() const {
+    if (Packs.size() == 0 ||
+        Packs[0].Saved.getKind() != clang::TemplateArgument::Pack)
+      return {};
+    unsigned PackSize = Packs[0].Saved.pack_size();
+
+    if (std::all_of(Packs.begin() + 1, Packs.end(), [&PackSize](const auto &P) {
+          return P.Saved.getKind() == TemplateArgument::Pack &&
+                 P.Saved.pack_size() == PackSize;
+        }))
+      return PackSize;
+    return {};
+  }
+
+  /// Determine whether this pack has already been deduced from a previous
+  /// argument.
+  bool isDeducedFromEarlierParameter() const {
+    return DeducedFromEarlierParameter;
+  }
+
   /// Determine whether this pack has already been partially expanded into a
   /// sequence of (prior) function parameters / template arguments.
   bool isPartiallyExpanded() { return IsPartiallyExpanded; }
@@ -1003,6 +1025,7 @@ class PackDeductionScope {
   unsigned PackElements = 0;
   bool IsPartiallyExpanded = false;
   bool DeducePackIfNotAlreadyDeduced = false;
+  bool DeducedFromEarlierParameter = false;
   /// The number of expansions, if we have a fully-expanded pack in this scope.
   std::optional<unsigned> FixedNumExpansions;
 
@@ -4371,6 +4394,34 @@ Sema::TemplateDeductionResult Sema::DeduceTemplateArguments(
           // corresponding argument is a list?
           PackScope.nextPackElement();
         }
+      } else if (!IsTrailingPack && !PackScope.isPartiallyExpanded() &&
+                 PackScope.isDeducedFromEarlierParameter()) {
+        // [temp.deduct.general#3]
+        // When all template arguments have been deduced
+        // or obtained from default template arguments, all uses of template
+        // parameters in the template parameter list of the template are
+        // replaced with the corresponding deduced or default argument values
+        //
+        // If we have a trailing parameter pack, that has been deduced
+        // previously we substitute the pack here in a similar fashion as
+        // above with the trailing parameter packs. The main difference here is
+        // that, in this case we are not processing all of the remaining
+        // arguments. We are only process as many arguments as much we have in
+        // the already deduced parameter.
+        std::optional<unsigned> ArgPosAfterSubstitution =
+            PackScope.getSavedPackSizeIfAllEqual();
+        if (!ArgPosAfterSubstitution)
+          continue;
+
+        unsigned PackArgEnd = ArgIdx + *ArgPosAfterSubstitution;
+        for (; ArgIdx < PackArgEnd && ArgIdx < Args.size(); ArgIdx++) {
+          ParamTypesForArgChecking.push_back(ParamPattern);
+          if (auto Result = DeduceCallArgument(ParamPattern, ArgIdx,
+                                               /*ExplicitObjetArgument=*/false))
+            return Result;
+
+          PackScope.nextPackElement();
+        }
       }
     }
 

clang/test/CXX/temp/temp.fct.spec/temp.deduct/temp.deduct.call/p1-0x.cpp

@@ -81,7 +81,7 @@ void test_pair_deduction(int *ip, float *fp, double *dp) {
 template<typename ...Types> struct tuple { };
 
 template<typename ...Types>
-void pack_not_at_end(tuple<Types...>, Types... values, int); // expected-note {{<int *, double *> vs. <>}}
+void pack_not_at_end(tuple<Types...>, Types... values, int); // expected-note {{<int *, double *> vs. <int, int>}}
 
 void test_pack_not_at_end(tuple<int*, double*> t2) {
   pack_not_at_end(t2, 0, 0, 0); // expected-error {{no match}}

clang/test/SemaTemplate/deduction.cpp

@@ -13,6 +13,17 @@ struct X0<int, A> {
   static const unsigned value = 1;
 };
 
+template<class T>
+struct type_identity {
+    using type = T;
+};
+
+template<class T>
+using type_identity_t = typename type_identity<T>::type;
+
+template <typename... T>
+struct args_tag {};
+
 template<int> struct X0i;
 template<long> struct X0l;
 int array_x0a[X0<long, X0l>::value == 0? 1 : -1];
@@ -431,6 +442,23 @@ namespace deduction_after_explicit_pack {
     i<int, int>(0, 1, 2, 3, 4, 5); // expected-error {{no match}}
   }
 
+  template <typename... T>
+  void bar(args_tag<T...>, type_identity_t<T>..., int mid, type_identity_t<T>...) {}
+  void call_bar() {
+    bar(args_tag<int, int>{}, 4, 8, 1001, 16, 23);
+  }
+
+  template <typename... Y, typename... T>
+  void foo(args_tag<Y...>, args_tag<T...>, type_identity_t<T>..., int mid, type_identity_t<T>...) {}
+  void call_foo() {
+    foo(args_tag<const int,const int, const int>{}, args_tag<int, int, int>{}, 4, 8, 9, 15, 16, 23, 1);
+  }
+
+  template <typename... Y, typename... T> void baz(args_tag<T...>, T..., T...) {}
+  void call_baz() {
+    baz(args_tag<int, int>{}, 1, 2, 3, 4);
+  }
+
   // GCC alarmingly accepts this by deducing T={int} by matching the second
   // parameter against the first argument, then passing the first argument
   // through the first parameter.