[clang][Sema] Avoid non-empty unexpanded pack assertion for FunctionParmPackExpr (#69224)

Closes #61460.

We have FunctionParmPackExpr that serves as the unexpanded expression
but from which the visitor collects none, which may lead to assertion
failure during the template instantiation.

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -553,6 +553,8 @@ Bug Fixes in This Version
   Fixes (`#67687 <https://github.com/llvm/llvm-project/issues/67687>`_)
 - Fix crash from constexpr evaluator evaluating uninitialized arrays as rvalue.
   Fixes (`#67317 <https://github.com/llvm/llvm-project/issues/67317>`_)
+- Fixed an issue that a benign assertion might hit when instantiating a pack expansion
+  inside a lambda. (`#61460 <https://github.com/llvm/llvm-project/issues/61460>`_)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaTemplateVariadic.cpp

@@ -402,6 +402,13 @@ bool Sema::DiagnoseUnexpandedParameterPack(Expr *E,
   if (!E->containsUnexpandedParameterPack())
     return false;
 
+  // CollectUnexpandedParameterPacksVisitor does not expect to see a
+  // FunctionParmPackExpr, but diagnosing unexpected parameter packs may still
+  // see such an expression in a lambda body.
+  // We'll bail out early in this case to avoid triggering an assertion.
+  if (isa<FunctionParmPackExpr>(E) && getEnclosingLambda())
+    return false;
+
   SmallVector<UnexpandedParameterPack, 2> Unexpanded;
   CollectUnexpandedParameterPacksVisitor(Unexpanded).TraverseStmt(E);
   assert(!Unexpanded.empty() && "Unable to find unexpanded parameter packs");

clang/test/SemaCXX/pr61460.cpp

@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -std=c++17 %s -fsyntax-only -verify
+
+template <typename... Ts> void g(Ts... p1s) {
+  (void)[&](auto... p2s) { ([&] { p1s; p2s; }, ...); };
+}
+
+void f1() {
+  g();
+}
+
+template <typename... Ts> void g2(Ts... p1s) {
+  (void)[&](auto... p2s) { [&] { p1s; p2s; }; }; // expected-error {{expression contains unexpanded parameter pack 'p2s'}}
+}