[clang] [Clang] Fix crash when visting a fold expression in a default argument (PR #67514)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik created https://github.com/llvm/llvm-project/pull/67514

CheckDefaultArgumentVisitor::Visit(...) assumes that the children of Expr will not be NULL. This is not a valid assumption and when we have a CXXFoldExpr the children can be NULL and this causes a crash.

Fixes: https://github.com/llvm/llvm-project/issues/67395

>From 2c8147f4f781ad8eaef188b294ce3588069f1c01 Mon Sep 17 00:00:00 2001
From: Shafik Yaghmour <shafik.yaghmour at intel.com>
Date: Tue, 26 Sep 2023 18:55:44 -0700
Subject: [PATCH] [Clang] Fix crash when visting a fold expression in a default
 argument

CheckDefaultArgumentVisitor::Visit(...) assumes that the children of Expr will
not be NULL. This is not a valid assumption and when we have a CXXFoldExpr
the children can be NULL and this causes a crash.

Fixes: https://github.com/llvm/llvm-project/issues/67395
---
 clang/lib/Sema/SemaDeclCXX.cpp                     | 3 ++-
 clang/test/SemaTemplate/cxx1z-fold-expressions.cpp | 8 ++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index 0091e0ecf6f3986..302e944d5d74f1c 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -86,7 +86,8 @@ class CheckDefaultArgumentVisitor
 bool CheckDefaultArgumentVisitor::VisitExpr(const Expr *Node) {
   bool IsInvalid = false;
   for (const Stmt *SubStmt : Node->children())
-    IsInvalid |= Visit(SubStmt);
+    if (SubStmt)
+      IsInvalid |= Visit(SubStmt);
   return IsInvalid;
 }
 
diff --git a/clang/test/SemaTemplate/cxx1z-fold-expressions.cpp b/clang/test/SemaTemplate/cxx1z-fold-expressions.cpp
index 518eaf0e05239e0..47a252eb335f6e5 100644
--- a/clang/test/SemaTemplate/cxx1z-fold-expressions.cpp
+++ b/clang/test/SemaTemplate/cxx1z-fold-expressions.cpp
@@ -124,3 +124,11 @@ namespace PR30738 {
   int test_h3 = h<struct X>(1, 2, 3);
   N::S test_h4 = h<struct X>(N::S(), N::S(), N::S()); // expected-note {{instantiation of}}
 }
+
+namespace GH67395 {
+template <typename>
+bool f();
+
+template <typename... T>
+void g(bool = (f<T>() || ...));
+}