[tmp] Enable partial += with <= support

src/zc/buildout/buildout.py

@@ -2311,10 +2311,17 @@ def _update(d1, d2):
         if section in d1:
             _update_section(d1[section], d2[section])
         else:
-            # In order to process += (and -=) correctly when
+            # XXX: In order to process += (and -=) correctly when
             # <key> = <value> and <key> += <value> are in the same section
-            # _update_section must be called even when section is not in d1
-            d1[section] = _update_section({}, d2[section])
+            # _update_section should be called even when section is not in d1
+            # Hack: When <= is used in the section, _update_section will be
+            # called later anyway, so we can avoid calling it now which will
+            # enable brittle and partial support for += (or -=) with keys that
+            # come from the <= sections.
+            # TODO: Either implement += and -= support with <= fully or call
+            # _update_section systematically and give up <= compatibility.
+            s2 = d2[section]
+            d1[section] = s2 if '<' in s2 else _update_section({}, s2)
     return d1
 
 def _recipe(options):