Make unused **keyword argument show up in locals() (GH-4899)

If you had a function
```
def f(**kwds):
  return locals()
```
then 'kwds' would not appear in locals. Found while investigating
one of the coverage gaps listed in
https://github.com/cython/cython/issues/4163.

```
allow_null = all(ref.node.allow_null for ref in self.starstar_arg.entry.cf_references)  # 3772 ↛ exit
if allow_null:    # 3773 ↛ 3774
    code.putln("%s = NULL;" % (self.starstar_arg.entry.cname,))
```
This uncovered code was wrong and has been removed. The only way
I can see to have an `allow_null` reference to the ** argument
would be to use `locals()`, and in this case an empty dict
should be generated.

2 files changed, 17 insertions, 9 deletions

diff --git a/Cython/Compiler/Nodes.py b/Cython/Compiler/Nodes.py
index 44c5f0f98..9c320cafa 100644
--- a/Cython/Compiler/Nodes.py
+++ b/Cython/Compiler/Nodes.py
@@ -3934,15 +3934,11 @@ class DefNodeWrapper(FuncDefNode):
                 self.starstar_arg.entry.cname, self.error_value()))
             code.put_gotref(self.starstar_arg.entry.cname, py_object_type)
             code.putln("} else {")
-            allow_null = all(ref.node.allow_null for ref in self.starstar_arg.entry.cf_references)
-            if allow_null:
-                code.putln("%s = NULL;" % (self.starstar_arg.entry.cname,))
-            else:
-                code.putln("%s = PyDict_New();" % (self.starstar_arg.entry.cname,))
-                code.putln("if (unlikely(!%s)) return %s;" % (
-                    self.starstar_arg.entry.cname, self.error_value()))
-                code.put_var_gotref(self.starstar_arg.entry)
-            self.starstar_arg.entry.xdecref_cleanup = allow_null
+            code.putln("%s = PyDict_New();" % (self.starstar_arg.entry.cname,))
+            code.putln("if (unlikely(!%s)) return %s;" % (
+                self.starstar_arg.entry.cname, self.error_value()))
+            code.put_var_gotref(self.starstar_arg.entry)
+            self.starstar_arg.entry.xdecref_cleanup = False
             code.putln("}")
 
         if self.self_in_stararg and not self.target.is_staticmethod:
diff --git a/tests/run/locals.pyx b/tests/run/locals.pyx
index 9473ad01e..2e7c0a961 100644
--- a/tests/run/locals.pyx
+++ b/tests/run/locals.pyx
@@ -5,6 +5,8 @@ def get_locals(x, *args, **kwds):
     """
     >>> sorted( get_locals(1,2,3, k=5).items() )
     [('args', (2, 3)), ('kwds', {'k': 5}), ('x', 1), ('y', 'hi'), ('z', 5)]
+    >>> sorted( get_locals(1).items() )  # args and kwds should *always* be present even if not passed
+    [('args', ()), ('kwds', {}), ('x', 1), ('y', 'hi'), ('z', 5)]
     """
     cdef int z = 5
     y = "hi"
@@ -14,6 +16,8 @@ def get_vars(x, *args, **kwds):
     """
     >>> sorted( get_vars(1,2,3, k=5).items() )
     [('args', (2, 3)), ('kwds', {'k': 5}), ('x', 1), ('y', 'hi'), ('z', 5)]
+    >>> sorted( get_vars(1).items() )
+    [('args', ()), ('kwds', {}), ('x', 1), ('y', 'hi'), ('z', 5)]
     """
     cdef int z = 5
     y = "hi"
@@ -23,6 +27,8 @@ def get_dir(x, *args, **kwds):
     """
     >>> sorted( get_dir(1,2,3, k=5) )
     ['args', 'kwds', 'x', 'y', 'z']
+    >>> sorted( get_dir(1) )
+    ['args', 'kwds', 'x', 'y', 'z']
     """
     cdef int z = 5
     y = "hi"
@@ -36,6 +42,8 @@ def in_locals(x, *args, **kwds):
     True
     >>> in_locals('X')
     False
+    >>> in_locals('kwds')
+    True
     """
     cdef int z = 5
     y = "hi"
@@ -49,6 +57,8 @@ def in_dir(x, *args, **kwds):
     True
     >>> in_dir('X')
     False
+    >>> in_dir('kwds')
+    True
     """
     cdef int z = 5
     y = "hi"
@@ -62,6 +72,8 @@ def in_vars(x, *args, **kwds):
     True
     >>> in_vars('X')
     False
+    >>> in_vars('kwds')
+    True
     """
     cdef int z = 5
     y = "hi"