[Clang][SemaCXX] Fix bug where unexpanded lambda captures where assumed to have size 1 (#101385)

Fixes #63677

Author: MitalAshok

clang/docs/ReleaseNotes.rst

@@ -185,6 +185,7 @@ Bug Fixes to C++ Support
 - Clang now correctly recognizes the correct context for parameter
   substitutions in concepts, so it doesn't incorrectly complain of missing
   module imports in those situations. (#GH60336)
+- Fix init-capture packs having a size of one before being instantiated. (#GH63677)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -1447,7 +1447,8 @@ namespace {
     }
 
     void transformedLocalDecl(Decl *Old, ArrayRef<Decl *> NewDecls) {
-      if (Old->isParameterPack()) {
+      if (Old->isParameterPack() &&
+          (NewDecls.size() != 1 || !NewDecls.front()->isParameterPack())) {
         SemaRef.CurrentInstantiationScope->MakeInstantiatedLocalArgPack(Old);
         for (auto *New : NewDecls)
           SemaRef.CurrentInstantiationScope->InstantiatedLocalPackArg(

clang/lib/Sema/TreeTransform.h

@@ -14340,14 +14340,14 @@ TreeTransform<Derived>::TransformLambdaExpr(LambdaExpr *E) {
               OldVD->getInit()->getSourceRange(), Unexpanded, Expand,
               RetainExpansion, NumExpansions))
         return ExprError();
+      assert(!RetainExpansion && "Should not need to retain expansion after a "
+                                 "capture since it cannot be extended");
       if (Expand) {
         for (unsigned I = 0; I != *NumExpansions; ++I) {
           Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), I);
           SubstInitCapture(SourceLocation(), std::nullopt);
         }
-      }
-      if (!Expand || RetainExpansion) {
-        ForgetPartiallySubstitutedPackRAII Forget(getDerived());
+      } else {
         SubstInitCapture(ExpansionTL.getEllipsisLoc(), NumExpansions);
         Result.EllipsisLoc = ExpansionTL.getEllipsisLoc();
       }

clang/test/SemaCXX/lambda-pack-expansion.cpp

@@ -41,3 +41,30 @@ int h(Ts... ts) {
 }
 
 }
+
+namespace GH63677 {
+
+template<typename>
+void f() {
+  []<typename... Ts>() -> void {
+    [...us = Ts{}]{
+      (Ts(us), ...);
+    };
+  }.template operator()<int, int>();
+}
+
+template void f<int>();
+
+template <class>
+inline constexpr auto fun =
+  []<class... Ts>(Ts... ts) {
+    return [... us = (Ts&&) ts]<class Fun>(Fun&& fn) mutable {
+      return static_cast<Fun&&>(fn)(static_cast<Ts&&>(us)...);
+    };
+  };
+
+void f() {
+  [[maybe_unused]] auto s = fun<int>(1, 2, 3, 4);
+}
+
+}