Fall back to unitary protocol in cirq.mixture for decomposable unitaries

[himmat05]

Fixes #8402

Description

cirq.has_mixture(val) returns True for decomposable unitary objects (such as Moment and CircuitOperation), but cirq.mixture(val) was raising TypeError because it only inspected val._unitary_ and never fell back to cirq.has_unitary(val) / cirq.unitary(val).

This PR aligns cirq.mixture with cirq.kraus by falling back to cirq.unitary(val) whenever cirq.has_unitary(val) is True.

Testing

• Added test_mixture_decomposable_unitary testing cirq.Moment and cirq.CircuitOperation.
• Ran all protocol tests in cirq-core/cirq/protocols/ (1228 passed).

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.59%. Comparing base (1403830) to head (056ae11).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8406   +/-   ##
=======================================
  Coverage   99.59%   99.59%           
=======================================
  Files        1133     1133           
  Lines      103919   103934   +15     
=======================================
+ Hits       103498   103513   +15     
  Misses        421      421           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[arettig]

Hello, thanks for looking into this. I have a couple minor comments below. The main thing is that now that mixture can build the unitary from the decomposition, we should not call that potentially slow function from has_mixture.

[arettig]

has_mixture should also be changed. Here, mixture will now form the full unitary via decomposition if possible, which can be very large/slow and disregards the allow_decompose parameter. Since the decomposition is checked above, this should change to just call _mixture_ here if it exists.

[arettig]

Please use pytest.mark.parameterize rather than a loop here.

[arettig]

Please update this docstring as well.

[himmat05]

Hi @arettig , thanks for the review!

I've addressed all the requested changes in the latest commit:

has_mixture: Replaced the fallback call to mixture(val, None) with directly checking mixture. This prevents constructing full unitaries via decomposition and properly honors the allow_decompose parameter. Added test_has_mixture_allow_decompose_false to verify this behavior.
mixture docstring: Updated the Returns: and Raises: sections to document the unitary fallback behavior, default value handling, and accurate TypeError conditions.
Test refactor: Parametrized test_mixture_decomposable_unitary using @pytest.mark.parametrize instead of a for loop.
All protocol tests and lint checks are passing.