[Clang] Fix crash when visting a fold expression in a default argument (#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

clang/docs/ReleaseNotes.rst

@@ -363,6 +363,10 @@ Bug Fixes to C++ Support
   in the enclosing expression of a lambda expression with a noexcept specifier.
   (`#67492 <https://github.com/llvm/llvm-project/issues/67492>`_)
 
+- Fix crash when fold expression was used in the initialization of default
+  argument. Fixes:
+  (`#67395 <https://github.com/llvm/llvm-project/issues/67395>`_)
+
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 - Fixed an import failure of recursive friend class template.

clang/lib/Sema/SemaDeclCXX.cpp

@@ -86,7 +86,8 @@ public:
 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;
 }
 

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>() || ...));
+}