Preserve default arguments values on methods for introspection whenever possible (GH-4118)

Closes https://github.com/cython/cython/issues/4061

Cython/Compiler/ExprNodes.py

@@ -9371,9 +9371,9 @@ class PyCFunctionNode(ExprNode, ModuleNameMixin):
         # so their optional arguments must be static, too.
         # TODO: change CyFunction implementation to pass both function object and owning object for method calls
         must_use_constants = env.is_c_class_scope or (self.def_node.is_wrapper and env.is_module_scope)
-
         for arg in self.def_node.args:
-            if arg.default and not must_use_constants:
+            if arg.default:
+                if not must_use_constants:
                     if not arg.default.is_literal:
                         arg.is_dynamic = True
                         if arg.type.is_pyobject:
@@ -9382,6 +9382,7 @@ class PyCFunctionNode(ExprNode, ModuleNameMixin):
                             nonliteral_other.append(arg)
                     else:
                         arg.default = DefaultLiteralArgNode(arg.pos, arg.default)
+                if arg.default.type and arg.default.type.can_coerce_to_pyobject(env):
                     if arg.kw_only:
                         default_kwargs.append(arg)
                     else:

tests/run/cyfunction_defaults.pyx

@@ -251,3 +251,51 @@ def test_func_default_scope_local():
         return arg
     print i  # genexprs don't leak
     return func
+
+cdef class C:
+    def f1(self, a, b=1, c=[]):
+        pass
+    def f2(self, a, b=1,/, c=[1]):
+        pass
+    def f3(self, a, /, b=1, *, c=[1]):
+        pass
+    cpdef f4(self, a, char*c=NULL):
+        pass
+    cpdef f5(self, a, str s = "123"):
+        pass
+    cpdef f6(self, a, int s = 4):
+        pass
+    cpdef f7(self, a, dict s = {'a':22}):
+        pass
+    cpdef f8(self, a, list s = [15]):
+        pass
+
+
+def check_defaults_on_methods_for_introspection():
+    """
+    >>> C.f1.__defaults__
+    (1, [])
+    >>> C.f1.__kwdefaults__
+    >>> C.f2.__defaults__
+    (1, [1])
+    >>> C.f2.__kwdefaults__
+    >>> C.f3.__defaults__
+    (1,)
+    >>> C.f3.__kwdefaults__
+    {'c': [1]}
+    >>> C.f4.__defaults__
+    >>> C.f4.__kwdefaults__
+    >>> C.f5.__defaults__
+    ('123',)
+    >>> C.f5.__kwdefaults__
+    >>> C.f6.__defaults__
+    (4,)
+    >>> C.f6.__kwdefaults__
+    >>> C.f7.__defaults__
+    ({'a': 22},)
+    >>> C.f7.__kwdefaults__
+    >>> C.f8.__defaults__
+    ([15],)
+    >>> C.f8.__kwdefaults__
+    """
+    pass

tests/run/py34_signature.pyx

@@ -101,19 +101,17 @@ cpdef cp1(a, b):
     """
 
 
-# Currently broken, see GH #1864
 cpdef cp2(a, b=True):
     """
     >>> def py_cp2(a, b=True): pass
 
-    #>>> signatures_match(cp2, py_cp2)
+    >>> signatures_match(cp2, py_cp2)
     """
 
 
-# Currently broken, see GH #1864
 cpdef cp3(a=1, b=True):
     """
     >>> def py_cp3(a=1, b=True): pass
 
-    #>>> signatures_match(cp3, py_cp3)
+    >>> signatures_match(cp3, py_cp3)
     """