Prevent fused dispatcher optional arguments being overwritten (GH-3519)

* Prevent fused dispatcher optional arguments being overwritten

If the 5th argument of the regular functions was optional it'd
overwrite the _fused_sigindex default of the dispatcher, causing
type errors at runtime.

closes https://github.com/cython/cython/issues/3511

Cython/Compiler/ExprNodes.py

@@ -9449,6 +9449,10 @@ class PyCFunctionNode(ExprNode, ModuleNameMixin):
         if self.defaults_tuple:
             code.putln('__Pyx_CyFunction_SetDefaultsTuple(%s, %s);' % (
                 self.result(), self.defaults_tuple.py_result()))
+        if not self.specialized_cpdefs:
+            # disable introspection functions for fused dispatcher function since the user never sees it
+            # TODO: this is mostly disabled because the attributes end up pointing to ones belonging
+            #  to the specializations - ideally this would be fixed instead
             if self.defaults_kwdict:
                 code.putln('__Pyx_CyFunction_SetDefaultsKwDict(%s, %s);' % (
                     self.result(), self.defaults_kwdict.py_result()))

Cython/Compiler/FusedNode.py

@@ -839,7 +839,8 @@ class FusedCFuncDefNode(StatListNode):
 
         for i, stat in enumerate(self.stats):
             stat = self.stats[i] = stat.analyse_expressions(env)
-            if isinstance(stat, FuncDefNode):
+            if isinstance(stat, FuncDefNode) and stat is not self.py_func:
+                # 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)

tests/run/fused_def.pyx

@@ -54,9 +54,13 @@ f = 5.6
 i = 9
 
 
-def opt_func(fused_t obj, cython.floating myf = 1.2, cython.integral myi = 7):
+def opt_func(fused_t obj, cython.floating myf = 1.2, cython.integral myi = 7,
+             another_opt = 2, yet_another_opt=3):
     """
-    Test runtime dispatch, indexing of various kinds and optional arguments
+    Test runtime dispatch, indexing of various kinds and optional arguments.
+    Use 5 arguments because at one point the optional argument from the
+    5th argument was overwriting that of the __pyx_fused dispatcher.
+    https://github.com/cython/cython/issues/3511
 
     >>> opt_func("spam", f, i)
     str object double long
@@ -121,9 +125,9 @@ def opt_func(fused_t obj, cython.floating myf = 1.2, cython.integral myi = 7):
     >>> opt_func()
     Traceback (most recent call last):
     TypeError: Expected at least 1 argument, got 0
-    >>> opt_func("abc", f, i, 5)  # doctest: +ELLIPSIS
+    >>> opt_func("abc", f, i, 5, 5, 5)  # doctest: +ELLIPSIS
     Traceback (most recent call last):
-    TypeError: ...at most 3...
+    TypeError: ...at most 5...
     >>> opt_func[ExtClassA, cy.float, cy.long](object(), f)
     Traceback (most recent call last):
     TypeError: Argument 'obj' has incorrect type (expected fused_def.ExtClassA, got object)
@@ -154,19 +158,19 @@ def test_opt_func():
 def test_opt_func_introspection():
     """
     >>> opt_func.__defaults__
-    (1.2, 7)
+    (1.2, 7, 2, 3)
     >>> opt_func.__kwdefaults__
     >>> opt_func.__annotations__
     {}
 
     >>> opt_func[str, float, int].__defaults__
-    (1.2, 7)
+    (1.2, 7, 2, 3)
     >>> opt_func[str, float, int].__kwdefaults__
     >>> opt_func[str, float, int].__annotations__
     {}
 
     >>> opt_func[str, cy.double, cy.long].__defaults__
-    (1.2, 7)
+    (1.2, 7, 2, 3)
     >>> opt_func[str, cy.double, cy.long].__kwdefaults__
     >>> opt_func[str, cy.double, cy.long].__annotations__
     {}