mypy type check errors with default argument to cirq protocols.

[tanujkhattar]

Description of the issue
This came up while reviewing #4881 and offline discussions with @Zshan0.

A common pattern used throughout cirq protocols is to take a default: TDefault parameter which should be returned if the protocol does not succeed, or raise a type error if no explicit default has been provided. For example:

Cirq/cirq-core/cirq/protocols/unitary_protocol.py

Lines 42 to 44 in 87d3632

	RaiseTypeErrorIfNotProvided: np.ndarray = np.array([])
	TDefault = TypeVar('TDefault')

Cirq/cirq-core/cirq/protocols/unitary_protocol.py

Lines 89 to 91 in 87d3632

	def unitary(
	val: Any, default: TDefault = RaiseTypeErrorIfNotProvided
	) -> Union[np.ndarray, TDefault]:

However, this construct actually raises a mypy error and is a known issue -- python/mypy#8739

How to reproduce the issue

Here is a minimal failing example that follows a similar construct:

TDefault = TypeVar('TDefault')
RaiseTypeErrorIfNotProvided: List = []

def unitary(val: Any, default: TDefault = RaiseTypeErrorIfNotProvided) -> Union[List, TDefault]:
    return default

https://mypy-play.net/?mypy=latest&python=3.10&gist=f60ce6c21997d6559f0bf5bbe0881230

Summary
There are two different issues here that need to fixed:

• Figure out and fix why ./check/mypy does not detect this error.
• Fix the type annotations on default parameter of protocols. (eg: by using the overload + optional signature for default).

Cirq version
0.14dev

[95-martin-orion]

Cirq cynq: not terribly high-priority, but we should at least confirm that mypy isn't skipping files/directories.