Remove incorrect dict unpacking optimisation that leaked external dict changes...

Remove incorrect dict unpacking optimisation that leaked external dict changes into the result (GH-4091)

Closes https://github.com/cython/cython/issues/3227

Cython/Compiler/ExprNodes.py

@@ -6669,22 +6669,13 @@ class MergedDictNode(ExprNode):
         return dict_type
 
     def analyse_types(self, env):
-        args = [
+        self.keyword_args = [
             arg.analyse_types(env).coerce_to_pyobject(env).as_none_safe_node(
                 # FIXME: CPython's error message starts with the runtime function name
                 'argument after ** must be a mapping, not NoneType')
             for arg in self.keyword_args
         ]
 
-        if len(args) == 1 and args[0].type is dict_type:
-            # strip this intermediate node and use the bare dict
-            arg = args[0]
-            if arg.is_name and arg.entry.is_arg and len(arg.entry.cf_assignments) == 1:
-                # passing **kwargs through to function call => allow NULL
-                arg.allow_null = True
-            return arg
-
-        self.keyword_args = args
         return self
 
     def may_be_none(self):

tests/run/dict.pyx

@@ -117,3 +117,22 @@ def item_creation_sideeffect(L, sideeffect, unhashable):
     [2, 4, 5]
     """
     return {1:2, sideeffect(2): 3, 3: 4, unhashable(4): 5, sideeffect(5): 6}
+
+
+def dict_unpacking_not_for_arg_create_a_copy():
+    """
+    >>> dict_unpacking_not_for_arg_create_a_copy()
+    [('a', 'modified'), ('b', 'original')]
+    [('a', 'original'), ('b', 'original')]
+    """
+    data = {'a': 'original', 'b': 'original'}
+
+    func = lambda: {**data}
+
+    call_once = func()
+    call_once['a'] = 'modified'
+
+    call_twice = func()
+
+    print(sorted(call_once.items()))
+    print(sorted(call_twice.items()))

tests/run/kwargs_passthrough.pyx

-cimport cython
+import cython
 
-
-@cython.test_fail_if_path_exists('//MergedDictNode')
+#@cython.test_fail_if_path_exists('//MergedDictNode')
 def wrap_passthrough(f):
     """
     >>> def f(a=1): return a
@@ -80,7 +79,7 @@ def wrap_passthrough_more(f):
     return wrapper
 
 
-@cython.test_fail_if_path_exists('//MergedDictNode')
+#@cython.test_fail_if_path_exists('//MergedDictNode')
 def wrap_passthrough2(f):
     """
     >>> def f(a=1): return a
@@ -99,7 +98,7 @@ def wrap_passthrough2(f):
     return wrapper
 
 
-@cython.test_fail_if_path_exists('//MergedDictNode')
+#@cython.test_fail_if_path_exists('//MergedDictNode')
 def wrap_modify(f):
     """
     >>> def f(a=1, test=2):
@@ -123,7 +122,7 @@ def wrap_modify(f):
     return wrapper
 
 
-@cython.test_fail_if_path_exists('//MergedDictNode')
+#@cython.test_fail_if_path_exists('//MergedDictNode')
 def wrap_modify_mix(f):
     """
     >>> def f(a=1, test=2):
@@ -175,7 +174,21 @@ def wrap_modify_func(f):
     return wrapper
 
 
-@cython.test_assert_path_exists('//MergedDictNode')
+def modify_in_function():
+    """
+    >>> modify_in_function()
+    {'foo': 'bar'}
+    {'foo': 'bar'}
+    """
+    def inner(**kwds):
+        kwds['foo'] = 'modified'
+    d = {'foo': 'bar'}
+    print(d)
+    inner(**d)
+    print(d)
+
+
+#@cython.test_assert_path_exists('//MergedDictNode')
 def wrap_modify_func_mix(f):
     """
     >>> def f(a=1, test=2):
@@ -203,12 +216,11 @@ def wrap_modify_func_mix(f):
     return wrapper
 
 
-@cython.test_fail_if_path_exists('//MergedDictNode')
+#@cython.test_fail_if_path_exists('//MergedDictNode')
 def wrap_reassign(f):
     """
     >>> def f(a=1, test=2):
     ...     return a, test
-
     >>> wrapped = wrap_reassign(f)
     >>> wrapped(1)
     CALLED
@@ -227,7 +239,7 @@ def wrap_reassign(f):
     return wrapper
 
 
-@cython.test_fail_if_path_exists('//MergedDictNode')
+#@cython.test_fail_if_path_exists('//MergedDictNode')
 def kwargs_metaclass(**kwargs):
     """
     >>> K = kwargs_metaclass()