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

[Younan Zhang via cfe-commits]

https://github.com/zyn0217 updated https://github.com/llvm/llvm-project/pull/69224

>From 880f271cbad4aacb7647bc402f636adf12ca1147 Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109 at gmail.com>
Date: Mon, 16 Oct 2023 22:50:08 +0800
Subject: [PATCH] [clang][Sema] Avoid non-empty unexpanded pack assertion for
 FunctionParmPackExpr

Closes https://github.com/llvm/llvm-project/issues/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.
---
 clang/docs/ReleaseNotes.rst             |  2 ++
 clang/lib/Sema/SemaTemplateVariadic.cpp | 13 +++++++++++++
 clang/test/SemaCXX/pr61460.cpp          | 13 +++++++++++++
 3 files changed, 28 insertions(+)
 create mode 100644 clang/test/SemaCXX/pr61460.cpp

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 4cc148905d4e130..66f2bc3efefa8bf 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -496,6 +496,8 @@ Bug Fixes in This Version
   Fixes (`#65143 <https://github.com/llvm/llvm-project/issues/65143>`_)
 - Fix crash in formatting the real/imaginary part of a complex lvalue.
   Fixes (`#69218 <https://github.com/llvm/llvm-project/issues/69218>`_)
+- 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
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaTemplateVariadic.cpp b/clang/lib/Sema/SemaTemplateVariadic.cpp
index dfcc78dafdc4c31..6ec7fb9ef331190 100644
--- a/clang/lib/Sema/SemaTemplateVariadic.cpp
+++ b/clang/lib/Sema/SemaTemplateVariadic.cpp
@@ -402,6 +402,19 @@ bool Sema::DiagnoseUnexpandedParameterPack(Expr *E,
   if (!E->containsUnexpandedParameterPack())
     return false;
 
+  // Exception: The `CollectUnexpandedParameterPacksVisitor` collects nothing
+  // from a FunctionParmPackExpr. In the context where the collector is being
+  // used such as `collectUnexpandedParameterPacks`, this type of expression
+  // is not expected to be collected.
+  //
+  // Nonetheless, this function for diagnosis is still called anyway during
+  // template instantiation, with an expression of such a type if we're inside a
+  // lambda with unexpanded parameters.
+  //
+  // Rule out this case to prevent the assertion failure.
+  if (isa<FunctionParmPackExpr>(E) && getEnclosingLambda())
+    return false;
+
   SmallVector<UnexpandedParameterPack, 2> Unexpanded;
   CollectUnexpandedParameterPacksVisitor(Unexpanded).TraverseStmt(E);
   assert(!Unexpanded.empty() && "Unable to find unexpanded parameter packs");
diff --git a/clang/test/SemaCXX/pr61460.cpp b/clang/test/SemaCXX/pr61460.cpp
new file mode 100644
index 000000000000000..471b1b39d23c2b7
--- /dev/null
+++ b/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'}}
+}