Fix fused cpdef default arguments (GH-4413)

A couple of things were going wrong:
* they're creating CloneNodes (but not requiring the contents of the clone of the clone node to be temp)
* assignment from a clone node generates cleanup code (which is against the general rules of a clone node), and also loses a reference via giveref
* cpdef functions cause a small memory leak (#4412) by assigning to their constants twice. This is unfortunately difficult to test for. With this patch we no longer leak, but still duplicate a little bit of work.

Cython/Compiler/ExprNodes.py

@@ -13814,6 +13814,12 @@ class CloneNode(CoercionNode):
     def generate_disposal_code(self, code):
         pass
 
+    def generate_post_assignment_code(self, code):
+        # if we're assigning from a CloneNode then it's "giveref"ed away, so it does
+        # need a matching incref (ideally this should happen before the assignment though)
+        if self.is_temp:  # should usually be true
+            code.put_incref(self.result(), self.ctype())
+
     def free_temps(self, code):
         pass
 

Cython/Compiler/FusedNode.py

@@ -841,7 +841,8 @@ class FusedCFuncDefNode(StatListNode):
         for arg in self.node.args:
             if arg.default:
                 arg.default = arg.default.analyse_expressions(env)
-                defaults.append(ProxyNode(arg.default))
+                # coerce the argument to temp since CloneNode really requires a temp
+                defaults.append(ProxyNode(arg.default.coerce_to_temp(env)))
             else:
                 defaults.append(None)
 
@@ -851,7 +852,7 @@ class FusedCFuncDefNode(StatListNode):
                 # the dispatcher specifically doesn't want its defaults overriding
                 for arg, default in zip(stat.args, defaults):
                     if default is not None:
-                        arg.default = CloneNode(default).coerce_to(arg.type, env)
+                        arg.default = CloneNode(default).analyse_expressions(env).coerce_to(arg.type, env)
 
         if self.py_func:
             args = [CloneNode(default) for default in defaults if default]

Cython/Compiler/Nodes.py

@@ -2350,9 +2350,11 @@ class FuncDefNode(StatNode, BlockNode):
     def generate_execution_code(self, code):
         code.mark_pos(self.pos)
         # Evaluate and store argument default values
-        for arg in self.args:
-            if not arg.is_dynamic:
-                arg.generate_assignment_code(code)
+        # skip this for wrappers since it's done by wrapped function
+        if not self.is_wrapper:
+            for arg in self.args:
+                if not arg.is_dynamic:
+                    arg.generate_assignment_code(code)
 
     #
     # Special code for the __getbuffer__ function

tests/run/fused_cpdef.pyx

-# cython: language_level=3
+# cython: language_level=3str
 # mode: run
 
 cimport cython
@@ -186,3 +186,26 @@ def test_ambiguousmatch():
     Traceback (most recent call last):
     TypeError: Function call with ambiguous argument types
     """
+
+# https://github.com/cython/cython/issues/4409
+# default arguments + fused cpdef were crashing
+cpdef literal_default(cython.integral x, some_string="value"):
+    return x, some_string
+
+cpdef mutable_default(cython.integral x, some_value=[]):
+    some_value.append(x)
+    return some_value
+
+def test_defaults():
+    """
+    >>> literal_default(1)
+    (1, 'value')
+    >>> literal_default(1, "hello")
+    (1, 'hello')
+    >>> mutable_default(1)
+    [1]
+    >>> mutable_default(2)
+    [1, 2]
+    >>> mutable_default(3,[])
+    [3]
+    """