Be resilient to Zeitwerk::NameError in on_unload callbacks

Fixes #208.

lib/zeitwerk/loader.rb

@@ -144,7 +144,13 @@ def unload
         end
 
         to_unload.each do |cpath, (abspath, (parent, cname))|
-          unless on_unload_callbacks.empty?
+          # We have to check cdef? in this condition. Reason is, constants whose
+          # file does not define them have to be kept in to_unload as explained
+          # in the implementation of on_file_autoloaded.
+          #
+          # If the constant is not defined, on_unload should not be triggered
+          # for it.
+          if !on_unload_callbacks.empty? && cdef?(parent, cname)
             value = parent.const_get(cname)
             run_on_unload_callbacks(cpath, value, abspath)
           end

test/lib/zeitwerk/test_reloading.rb

@@ -189,4 +189,33 @@ module Namespace; end
       assert_equal 2, $test_eager_load_after_reload
     end
   end
+
+  test "reload recovers from name errors (w/o on_unload callbacks)" do
+    on_teardown { remove_const :Y }
+
+    files = [["x.rb", "Y = :typo"]]
+    with_setup(files) do
+      assert_raises(Zeitwerk::NameError) { X }
+
+      loader.reload
+      File.write("x.rb", "X = true")
+
+      assert X
+    end
+  end
+
+  test "reload recovers from name errors (w/ on_unload callbacks)" do
+    on_teardown { remove_const :Y }
+
+    files = [["x.rb", "Y = :typo"]]
+    with_setup(files) do
+      loader.on_unload {}
+      assert_raises(Zeitwerk::NameError) { X }
+
+      loader.reload
+      File.write("x.rb", "X = true")
+
+      assert X
+    end
+  end
 end