Fix signatures when used for Generic classes, fixes #1468

jedi/inference/value/klass.py

@@ -133,8 +133,6 @@ class ClassMixin(object):
 
     def py__call__(self, arguments=None):
         from jedi.inference.value import TreeInstance
-        if arguments is None:
-            arguments = ValuesArguments([])
         return ValueSet([TreeInstance(self.inference_state, self.parent_context, self, arguments)])
 
     def py__class__(self):
@@ -207,7 +205,11 @@ class ClassMixin(object):
             type_ = builtin_from_name(self.inference_state, u'type')
             assert isinstance(type_, ClassValue)
             if type_ != self:
-                for instance in type_.py__call__():
+                # We are not using execute_with_values here, because the
+                # plugin function for type would get executed instead of an
+                # instance creation.
+                args = ValuesArguments([])
+                for instance in type_.py__call__(args):
                     instance_filters = instance.get_filters()
                     # Filter out self filters
                     next(instance_filters)
@@ -215,7 +217,11 @@ class ClassMixin(object):
                     yield next(instance_filters)
 
     def get_signatures(self):
-        init_funcs = self.py__call__().py__getattribute__('__init__')
+        # Since calling staticmethod without a function is illegal, the Jedi
+        # plugin doesn't return anything. Therefore call directly and get what
+        # we want: An instance of staticmethod.
+        args = ValuesArguments([])
+        init_funcs = self.py__call__(args).py__getattribute__('__init__')
         return [sig.bind(self) for sig in init_funcs.get_signatures()]
 
     def _as_context(self):

test/test_inference/test_signature.py

@@ -113,6 +113,8 @@ def test_tree_signature(Script, environment, code, expected):
         # Classes / inheritance
         ('full_redirect(C)', 'z, *, c'),
         ('full_redirect(C())', 'y'),
+        ('full_redirect(G)', 't: T'),
+        ('full_redirect(G[str])', 't: T'),
         ('D', 'D(a, z, /)'),
         ('D()', 'D(x, y)'),
         ('D().foo', 'foo(a, *, bar, z, **kwargs)'),
@@ -189,6 +191,11 @@ def test_nested_signatures(Script, environment, combination, expected, skip_pre_
 
             def foo(self, a, **kwargs):
                 super().foo(**kwargs)
+
+        from typing import Generic, TypeVar
+        T = TypeVar('T')
+        class G(Generic[T]):
+            def __init__(self, i, t: T): ...
     ''')
     code += 'z = ' + combination + '\nz('
     sig, = Script(code).find_signatures()