Bug report

Bug summary

The code from https://sourceforge.net/p/matplotlib/mailman/attachment/CAG%3DuJ%2Bnw2dE05P9TOXTz_zp-mGP3cY801vMH7yt6vgP9_WzU8w%40mail.gmail.com/1/ works with version 1.5.3, i.e. a dashed arrow can be drawn with a solid arrowhead using matplotlib.patheffects.Stroke. This no longer works with version 2.0.2. I haven't tried version 2.1.0

Code for reproduction
For the code to run replace

class ConditionalStroke(pe._Base):

with

class ConditionalStroke(pe.RendererBase):

Actual outcome

With matplotlib-2.0.2 the code does not fail but the arrowhead portion of the arrow is drawn with the dashed linestyle. With matplotlib-1.5.3 if I replace pe.Stroke(linestyle="solid") in the code with pe.Stroke(linestyle="junk") I get

valueError: Unrecognized linestyle: junk

However, with the same replacement and running the code with matplotlib-2.0.2 there is no valueError and the arrow and arrowhead are drawn with the dashed linestyle.

Expected outcome
The desired result is to have an arrow with a dashed linestyle but with a solid arrowhead. It appears that matplotlib.patheffects.Stoke in version 2.0.2 is aware of the linestyle parameter but does not do anything with it. Pehaps with the newer matplotlib the intent is to not allow the linestyle to be changed via patheffects? If so, then perhaps an error should be thrown when the linestyle parameter in used. This is a feature that has been used in Sage (https://trac.sagemath.org/ticket/23696).

Matplotlib version

• Operating system: Gentoo
• Matplotlib version: 1.5.3 & 2.0.2
• Matplotlib backend (print(matplotlib.get_backend())): Qt4Agg & TkAgg
• Python version: 2.7.12
• Jupyter version (if applicable):
• Other libraries:
  Matplotlib was installed from source.