[Clang][P1061] Handle explicitly capturing binding packs

Author: zwuis

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -6232,8 +6232,16 @@ NamedDecl *Sema::FindInstantiatedDecl(SourceLocation Loc, NamedDecl *D,
     // declarations to their instantiations.
     if (CurrentInstantiationScope) {
       if (auto Found = CurrentInstantiationScope->findInstantiationOf(D)) {
-        if (Decl *FD = Found->dyn_cast<Decl *>())
+        if (Decl *FD = Found->dyn_cast<Decl *>()) {
+          if (auto *BD = dyn_cast<BindingDecl>(FD);
+              BD && BD->isParameterPack() &&
+              ArgumentPackSubstitutionIndex != -1) {
+            auto *DRE = cast<DeclRefExpr>(
+                BD->getBindingPackExprs()[ArgumentPackSubstitutionIndex]);
+            return cast<NamedDecl>(DRE->getDecl());
+          }
           return cast<NamedDecl>(FD);
+        }
 
         int PackIdx = ArgumentPackSubstitutionIndex;
         assert(PackIdx != -1 &&

clang/lib/Sema/TreeTransform.h

@@ -15298,12 +15298,11 @@ TreeTransform<Derived>::TransformLambdaExpr(LambdaExpr *E) {
         // The transform has determined that we should perform an expansion;
         // transform and capture each of the arguments.
         // expansion of the pattern. Do so.
-        auto *Pack = cast<VarDecl>(C->getCapturedVar());
+        auto *Pack = cast<ValueDecl>(C->getCapturedVar());
         for (unsigned I = 0; I != *NumExpansions; ++I) {
           Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), I);
-          VarDecl *CapturedVar
-            = cast_or_null<VarDecl>(getDerived().TransformDecl(C->getLocation(),
-                                                               Pack));
+          ValueDecl *CapturedVar = cast_if_present<ValueDecl>(
+              getDerived().TransformDecl(C->getLocation(), Pack));
           if (!CapturedVar) {
             Invalid = true;
             continue;

clang/test/SemaCXX/cxx2c-binding-pack.cpp

@@ -115,12 +115,24 @@ void decompose_bit_field() {
   int d = x...[3];
 }
 
+template <typename T>
+void lambda_capture() {
+  auto [...x] = T{};
+  [=] { (void)sum(x...); }();
+  [&] { (void)sum(x...); }();
+  [x...] { (void)sum(x...); }();
+  [&x...] { (void)sum(x...); }();
+}
+
 int main() {
   decompose_array<int>();
   decompose_tuple<fake_tuple>();
   decompose_struct<my_struct>();
   S<1, 2, 3, 4>::N<5, 6>().foo();
   decompose_bit_field<bit_fields>();
+  lambda_capture<int[5]>();
+  lambda_capture<fake_tuple>();
+  lambda_capture<my_struct>();
 }
 
 // P1061R10 Stuff