Fix: Throw error when attempting to dispatch on literal

runtype/dispatch.py

@@ -177,6 +177,10 @@ def define_function(self, f):
         for signature in get_func_signatures(self.typesystem, f):
             node = self.root
             for t in signature:
+                if not isinstance(t, type):
+                    # XXX this is a temporary fix for preventing certain types from being used for dispatch
+                    if not getattr(t, 'ALLOW_DISPATCH', True):
+                        raise ValueError(f"Type {t} cannot be used for dispatch")
                 node = node.follow_type[t]
 
             if node.func is not None:

runtype/pytypes.py

@@ -200,6 +200,8 @@ def test_instance(self, obj, sampler=None):
 class OneOf(PythonType):
     values: typing.Sequence
 
+    ALLOW_DISPATCH = False
+
     def __init__(self, values):
         self.values = values
 
@@ -218,6 +220,7 @@ def cast_from(self, obj):
             raise TypeMismatchError(obj, self)
 
 
+
 class GenericType(base_types.GenericType, PythonType):
     base: PythonDataType
     item: PythonType
@@ -448,6 +451,8 @@ def cast_from(self, obj):
 
 
 class _NoneType(OneOf):
+    ALLOW_DISPATCH = True   # Make an exception
+
     def __init__(self):
         super().__init__([None])
 

tests/test_basic.py

@@ -728,6 +728,24 @@ def f(t: Tree[int]):
 
         f(Tree())
 
+    def test_literal_dispatch(self):
+        try:
+            @multidispatch
+            def f(x: typing.Literal[1]):
+                return 1
+
+            @multidispatch
+            def f(x: typing.Literal[2]):
+                return 2
+        except ValueError:
+            pass
+        else:
+            assert False
+
+        # If it was working..
+        # assert f(1) == 1
+        # assert f(2) == 2
+
 
 class TestDataclass(TestCase):
     def setUp(self):