When I call a function decorated with functools.singledispatch without an argument, I get the following:

$ python
Python 3.6.5 (default, Jun 17 2018, 12:13:06)
[GCC 4.2.1 Compatible Apple LLVM 9.1.0 (clang-902.0.39.2)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import functools
>>> @functools.singledispatch
... def f(x):
...     pass
...
>>> f()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/Cellar/python/3.6.5_1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/functools.py", line 803, in wrapper
    return dispatch(args[0].__class__)(*args, **kw)
IndexError: tuple index out of range

I would have expected a TypeError along the lines of

TypeError: f() missing 1 required positional argument: 'x'

@doerwalter

Can I take a look this issue?

Signature mismatch should raise TypeError, not IndexError.
I think this should be backported to 3.6 and 3.7.

New changeset 445f1b35ce8461268438c8a6b327ddc764287e05 by INADA Naoki (Dong-hee Na) in branch 'master':
bpo-33967: Fix singledispatch raised IndexError when no args (GH-8184)
https://github.com/python/cpython/commit/445f1b35ce8461268438c8a6b327ddc764287e05

New changeset df9f633f94e97fc43e0235cb2be076491ea7f67f by Miss Islington (bot) in branch '3.7':
bpo-33967: Fix singledispatch raised IndexError when no args (GH-8184)
https://github.com/python/cpython/commit/df9f633f94e97fc43e0235cb2be076491ea7f67f

New changeset 6ceab46a602c6653356d67d7479391fba0b35697 by Miss Islington (bot) in branch '3.6':
bpo-33967: Fix singledispatch raised IndexError when no args (GH-8184)
https://github.com/python/cpython/commit/6ceab46a602c6653356d67d7479391fba0b35697

The test added in this change uses the deprecated method assertRaisesRegexp(), so -Werror will cause test_functools to fail. PR 8261 fixes this.

New changeset 9e9b2c32a34594e901b5b9a03c561a2a2bf63ece by INADA Naoki (Zackery Spytz) in branch 'master':
bpo-33967: Remove use of deprecated assertRaisesRegexp() (GH-8261)
https://github.com/python/cpython/commit/9e9b2c32a34594e901b5b9a03c561a2a2bf63ece

New changeset 336c9539a90e0f5df5bcc5b444b8400a58820583 by Victor Stinner (Miss Islington (bot)) in branch '3.6':
bpo-33967: Remove use of deprecated assertRaisesRegexp() (GH-8261) (GH-8295)
https://github.com/python/cpython/commit/336c9539a90e0f5df5bcc5b444b8400a58820583

New changeset 0a820a203032e3b1b9413afba45acd8f79f27f81 by Victor Stinner (Miss Islington (bot)) in branch '3.7':
bpo-33967: Remove use of deprecated assertRaisesRegexp() (GH-8261) (GH-8296)
https://github.com/python/cpython/commit/0a820a203032e3b1b9413afba45acd8f79f27f81

The purpose of using assertRaisesRegexp() was testing the error message. assertRaises() doesn't do this. Needed to restore the appropriate assertion method, but using a non-deprecated alias: assertRaisesRegex().

New changeset 56d8f57b83a37b05a6f2fbc3e141bbc1ba6cb3a2 by INADA Naoki in branch 'master':
bpo-33967: Fix wrong use of assertRaises (GH-8306)
https://github.com/python/cpython/commit/56d8f57b83a37b05a6f2fbc3e141bbc1ba6cb3a2

New changeset 892df9d15aae08d4033faeb34698e3c550c85854 by Miss Islington (bot) in branch '3.7':
bpo-33967: Fix wrong use of assertRaises (GH-8306)
https://github.com/python/cpython/commit/892df9d15aae08d4033faeb34698e3c550c85854

New changeset 8b5d191386350d28a0f20283dcb366cf50f82b97 by Miss Islington (bot) in branch '3.6':
bpo-33967: Fix wrong use of assertRaises (GH-8306)
https://github.com/python/cpython/commit/8b5d191386350d28a0f20283dcb366cf50f82b97