3 files changed, 16 insertions, 6 deletions
diff --git a/clang/lib/Sema/Sema.cpp b/clang/lib/Sema/Sema.cpp
index e07f60989d8..45fd2d3249d 100644
--- a/clang/lib/Sema/Sema.cpp
+++ b/clang/lib/Sema/Sema.cpp
@@ -1393,8 +1393,8 @@ void Sema::PopFunctionScopeInfo(const AnalysisBasedWarnings::Policy *WP,
     delete Scope;
 }
 
-void Sema::PushCompoundScope() {
-  getCurFunction()->CompoundScopes.push_back(CompoundScopeInfo());
+void Sema::PushCompoundScope(bool IsStmtExpr) {
+  getCurFunction()->CompoundScopes.push_back(CompoundScopeInfo(IsStmtExpr));
 }
 
 void Sema::PopCompoundScope() {
diff --git a/clang/lib/Sema/SemaStmt.cpp b/clang/lib/Sema/SemaStmt.cpp
index 1ebc36716a8..93907038e05 100644
--- a/clang/lib/Sema/SemaStmt.cpp
+++ b/clang/lib/Sema/SemaStmt.cpp
@@ -337,8 +337,8 @@ void Sema::DiagnoseUnusedExprResult(const Stmt *S) {
   DiagRuntimeBehavior(Loc, nullptr, PDiag(DiagID) << R1 << R2);
 }
 
-void Sema::ActOnStartOfCompoundStmt() {
-  PushCompoundScope();
+void Sema::ActOnStartOfCompoundStmt(bool IsStmtExpr) {
+  PushCompoundScope(IsStmtExpr);
 }
 
 void Sema::ActOnFinishOfCompoundStmt() {
diff --git a/clang/lib/Sema/SemaTemplateVariadic.cpp b/clang/lib/Sema/SemaTemplateVariadic.cpp
index d81837dad50..7aa0e317df0 100644
--- a/clang/lib/Sema/SemaTemplateVariadic.cpp
+++ b/clang/lib/Sema/SemaTemplateVariadic.cpp
@@ -314,8 +314,18 @@ Sema::DiagnoseUnexpandedParameterPacks(SourceLocation Loc,
   // later.
   SmallVector<UnexpandedParameterPack, 4> LambdaParamPackReferences;
   for (unsigned N = FunctionScopes.size(); N; --N) {
-    if (sema::LambdaScopeInfo *LSI =
-          dyn_cast<sema::LambdaScopeInfo>(FunctionScopes[N-1])) {
+    sema::FunctionScopeInfo *Func = FunctionScopes[N-1];
+    // We do not permit pack expansion that would duplicate a statement
+    // expression, not even within a lambda.
+    // FIXME: We could probably support this for statement expressions that do
+    // not contain labels, and for pack expansions that expand both the stmt
+    // expr and the enclosing lambda.
+    if (std::any_of(
+            Func->CompoundScopes.begin(), Func->CompoundScopes.end(),
+            [](sema::CompoundScopeInfo &CSI) { return CSI.IsStmtExpr; }))
+      break;
+
+    if (auto *LSI = dyn_cast<sema::LambdaScopeInfo>(Func)) {
       if (N == FunctionScopes.size()) {
         for (auto &Param : Unexpanded) {
           auto *PD = dyn_cast_or_null<ParmVarDecl>(