[clang] Fix AST IgnoreUnlessSpelledInSource traversal nullptr dereference (PR #146103)

[Jonathan Marriott via cfe-commits]

https://github.com/JonathanMarriott created https://github.com/llvm/llvm-project/pull/146103

Fixes #146101, in summary dumping a `catch(...)` statement using IgnoreUnlessSpelledInSource AST traversal causes a seg fault, as the variable declaration of the catch is `nullptr`.

Diagnosed the cause by attaching the debugger to `clang-query`, this PR adds a fix to check for `nullptr` before accessing the `isImplicit()` method of the `Decl` pointee in the AST node traverser visitor

>From 646d006e327f9d891dea5d9c334e44174a5560a3 Mon Sep 17 00:00:00 2001
From: Jon Marriott <jmarriott13 at bloomberg.net>
Date: Wed, 25 Jun 2025 18:04:59 +0100
Subject: [PATCH 1/2] Add check for decl nullptr to `ASTNodeTraverser::Visit`

---
 clang/include/clang/AST/ASTNodeTraverser.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/clang/include/clang/AST/ASTNodeTraverser.h b/clang/include/clang/AST/ASTNodeTraverser.h
index 8d02a50e2e8a5..8ebabb2bde10d 100644
--- a/clang/include/clang/AST/ASTNodeTraverser.h
+++ b/clang/include/clang/AST/ASTNodeTraverser.h
@@ -99,7 +99,7 @@ class ASTNodeTraverser
   TraversalKind GetTraversalKind() const { return Traversal; }
 
   void Visit(const Decl *D, bool VisitLocs = false) {
-    if (Traversal == TK_IgnoreUnlessSpelledInSource && D->isImplicit())
+    if (Traversal == TK_IgnoreUnlessSpelledInSource && D && D->isImplicit())
       return;
 
     getNodeDelegate().AddChild([=] {

>From 5c9c8b8d5a49fd7c10cc08554e36ac4744f0ee4f Mon Sep 17 00:00:00 2001
From: Jon Marriott <jmarriott13 at bloomberg.net>
Date: Thu, 26 Jun 2025 17:14:57 +0100
Subject: [PATCH 2/2] Add test

---
 clang/unittests/AST/ASTTraverserTest.cpp | 75 ++++++++++++++++++++++++
 1 file changed, 75 insertions(+)

diff --git a/clang/unittests/AST/ASTTraverserTest.cpp b/clang/unittests/AST/ASTTraverserTest.cpp
index 8b6e3e90c0ea6..988e81d8e51de 100644
--- a/clang/unittests/AST/ASTTraverserTest.cpp
+++ b/clang/unittests/AST/ASTTraverserTest.cpp
@@ -28,6 +28,10 @@ class NodeTreePrinter : public TextTreeStructure {
       : TextTreeStructure(OS, /* showColors */ false), OS(OS) {}
 
   void Visit(const Decl *D) {
+    if (!D) {
+      OS << "<<<NULL>>>";
+      return;
+    }
     OS << D->getDeclKindName() << "Decl";
     if (auto *ND = dyn_cast<NamedDecl>(D)) {
       OS << " '" << ND->getDeclName() << "'";
@@ -1932,4 +1936,75 @@ CXXRewrittenBinaryOperator
   }
 }
 
+TEST(Traverse, CatchStatements) {
+
+  auto AST = buildASTFromCode(R"cpp(
+void test()
+{
+  try
+  {
+    int a;
+  }
+  catch (...)
+  {
+    int b;
+  }
+
+  try
+  {
+    int a;
+  }
+  catch (const int&)
+  {
+    int b;
+  }
+}
+)cpp");
+
+  auto BN =
+      ast_matchers::match(cxxCatchStmt().bind("catch"), AST->getASTContext());
+  EXPECT_EQ(BN.size(), 2u);
+  const auto *catchWithoutDecl = BN[0].getNodeAs<Stmt>("catch");
+
+  llvm::StringRef Expected = R"cpp(
+CXXCatchStmt
+|-<<<NULL>>>
+`-CompoundStmt
+  `-DeclStmt
+    `-VarDecl 'b'
+)cpp";
+  EXPECT_EQ(dumpASTString(TK_AsIs, catchWithoutDecl), Expected);
+
+  Expected = R"cpp(
+CXXCatchStmt
+|-<<<NULL>>>
+`-CompoundStmt
+  `-DeclStmt
+    `-VarDecl 'b'
+)cpp";
+  EXPECT_EQ(dumpASTString(TK_IgnoreUnlessSpelledInSource, catchWithoutDecl),
+            Expected);
+
+  const auto *catchWithDecl = BN[1].getNodeAs<Stmt>("catch");
+
+  Expected = R"cpp(
+CXXCatchStmt
+|-VarDecl ''
+`-CompoundStmt
+  `-DeclStmt
+    `-VarDecl 'b'
+)cpp";
+  EXPECT_EQ(dumpASTString(TK_AsIs, catchWithDecl), Expected);
+
+  Expected = R"cpp(
+CXXCatchStmt
+|-VarDecl ''
+`-CompoundStmt
+  `-DeclStmt
+    `-VarDecl 'b'
+)cpp";
+  EXPECT_EQ(dumpASTString(TK_IgnoreUnlessSpelledInSource, catchWithDecl),
+            Expected);
+}
+
 } // namespace clang