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

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

Fixes #146101

Author: JonathanMarriott

clang/docs/ReleaseNotes.rst

@@ -903,6 +903,7 @@ Bug Fixes to AST Handling
 - Fixed dependency calculation for TypedefTypes (#GH89774)
 - The ODR checker now correctly hashes the names of conversion operators. (#GH143152)
 - Fixed the right parenthesis source location of ``CXXTemporaryObjectExpr``. (#GH143711)
+- Fixed a crash when performing an ``IgnoreUnlessSpelledInSource`` traversal of ASTs containing ``catch(...)`` statements. (#GH146103)
 
 Miscellaneous Bug Fixes
 ^^^^^^^^^^^^^^^^^^^^^^^

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([=] {

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