BUG: derivative of atan2 not defined about firedrake HOT 7 CLOSED

Oh, I see. The MathFunction handler assumes there is only one argument.

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Those PRs have landed and I have updated our UFL fork. I believe this is now fixed.

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We've just hit this too, looks like there are a couple of causes:

1. The ufl.utils.formatting.camel2undercase function still translates Atan2 -> atan_2, so the differentation handler isn't set up to a valid function.
2. TSFC doesn't set up the correct multifunction handler (it's still called atan_2 over there: https://github.com/firedrakeproject/tsfc/blob/b378c0879fea5a7cc88cabf9eae6c52b9921c34b/tsfc/ufl2gem.py#L81-L82)

This seems to be a sufficient patch to catch the missing changes:

diff --git a/ufl/utils/formatting.py b/ufl/utils/formatting.py
index d35164be..4df63cd9 100644
--- a/ufl/utils/formatting.py
+++ b/ufl/utils/formatting.py
@@ -11,7 +11,7 @@ def camel2underscore(name):
     letters = []
     lastlower = False
     for i in name:
-        thislower = i.islower()
+        thislower = i.islower() or i.isdigit()
         if not thislower:
             # Don't insert _ between multiple upper case letters
             if lastlower:
diff --git a/tsfc/ufl2gem.py b/tsfc/ufl2gem.py
index fc1c760..e283fb1 100644
--- a/tsfc/ufl2gem.py
+++ b/tsfc/ufl2gem.py
@@ -78,7 +78,7 @@ class Mixin(object):
     def math_function(self, o, expr):
         return MathFunction(o._name, expr)
 
-    def atan_2(self, o, y, x):
+    def atan2(self, o, y, x):
         return MathFunction("atan2", y, x)
 
     def bessel_i(self, o, nu, arg):

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I think the TSFC thing is a red herring, because atan2 will get caught by the generic MathFunction handler on line 78. We should probably delete the atan_2 handler.

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Curious, I needed that fix after making the change in UFL. There's a traceback in g-adopt/g-adopt#22 (comment) showing the result with only the UFL fix. It does seem to make more sense to use the generic handler, however.

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FEniCS/ufl#216

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also firedrakeproject/tsfc#299

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