FlippedInterface should propagate flipped to subinterfaces.

[cr1901]

Fixes #950.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e55dec9) 83.79% compared to head (4ee09b2) 83.81%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #954      +/-   ##
==========================================
+ Coverage   83.79%   83.81%   +0.02%     
==========================================
  Files          54       54              
  Lines        7768     7766       -2     
  Branches     1906     1905       -1     
==========================================
  Hits         6509     6509              
+ Misses       1053     1051       -2     
  Partials      206      206              

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[wanda-phi]

I'm not convinced a direct check for Interface is correct here — an interface doesn't actually have to inherit from the Interface class (and, for one, FlippedInterface doesn't), so this could break when a fancy Signature subclass with a custom interface class is involved.

A better check might be to check if 1) the name is in the signature, and 2) the relevant signature member is itself a signature.

[cr1901]

Improved added test after feedback from @wanda-phi (tyvm!).