gh-148306: Fix dis.distb() crash when traceback is None

Author: Ignyra

Lib/dis.py

@@ -142,11 +142,16 @@ def distb(tb=None, *, file=None, show_caches=False, adaptive=False, show_offsets
     if tb is None:
         try:
             if hasattr(sys, 'last_exc'):
-                tb = sys.last_exc.__traceback__
+                exc = sys.last_exc
+                tb = exc.__traceback__
+                if isinstance(exc, SyntaxError):
+                    raise RuntimeError("can't disassemble a SyntaxError")
             else:
                 tb = sys.last_traceback
         except AttributeError:
             raise RuntimeError("no last traceback to disassemble") from None
+
+
         while tb.tb_next: tb = tb.tb_next
     disassemble(tb.tb_frame.f_code, tb.tb_lasti, file=file, show_caches=show_caches, adaptive=adaptive, show_offsets=show_offsets, show_positions=show_positions)
 

Lib/test/test_dis.py

@@ -2478,6 +2478,16 @@ def test_distb_empty(self):
         with self.assertRaises(RuntimeError):
             dis.distb()
 
+    def test_distb_syntax_error(self):
+        try:
+            compile("???", "", "exec")
+        except SyntaxError as e:
+            sys.last_exc = e
+            sys.last_exc.__traceback__ = None
+
+        with self.assertRaises(RuntimeError):
+            dis.distb()
+
     def test_distb_last_traceback(self):
         self.maxDiff = None
         # We need to have an existing last traceback in `sys`: