FIX : Add test to document corner case

python/GafferSceneTest/ShaderTweakProxyTest.py

@@ -142,6 +142,47 @@ def test( self ) :
 		with self.assertRaisesRegex( Gaffer.ProcessException, 'Cannot use "texture1" in ShaderTweakProxy when tweaking "texture2", this would create cycle in shader network' ):
 			tweaks["out"].attributes( "/plane" )
 
+	def testAutoProxyValueTransferToComponent( self ) :
+
+		plane = GafferScene.Plane()
+		shader = GafferSceneTest.TestShader( "surface" )
+		shader["type"].setValue( "surface" )
+		shader["parameters"]["i"].setValue( 42 )
+
+		planeFilter = GafferScene.PathFilter()
+		planeFilter["paths"].setValue( IECore.StringVectorData( [ "/plane" ] ) )
+
+		assignment = GafferScene.ShaderAssignment()
+		assignment["in"].setInput( plane["out"] )
+		assignment["filter"].setInput( planeFilter["out"] )
+		assignment["shader"].setInput( shader["out"] )
+
+		tweakShader = GafferSceneTest.TestShader( "tweakShader" )
+
+		tweaks = GafferScene.ShaderTweaks()
+		tweaks["in"].setInput( assignment["out"] )
+		tweaks["filter"].setInput( planeFilter["out"] )
+		tweaks["shader"].setValue( "surface" )
+
+		tweaks["tweaks"].addChild( Gaffer.TweakPlug( "i", Gaffer.IntPlug() ) )
+		tweaks["tweaks"][0]["value"].setInput( tweakShader["out"]["r"] )
+
+		autoProxy = GafferScene.ShaderTweakProxy()
+		autoProxy.setupAutoProxy( Gaffer.IntPlug() )
+
+		tweakShader["parameters"]["c"]["g"].setInput( autoProxy["out"]["auto"] )
+
+		# This is quite a special case - there is no input to the parameter we are tweaking, so there is no
+		# connection to transfer, so we would expect the auto proxy to transfer the value - however the auto
+		# proxy output is connected to a subcomponent.
+		#
+		# The correct result is that the green component of tweakShader.c should be set to 42, transferring the value
+		# that was set. However, we have not yet added support for this fairly obscure case, so instead this test
+		# documents the current behaviour, which is to throw a semi-helpful exception.
+
+		with self.assertRaisesRegex( Gaffer.ProcessException, 'CompoundData has no child named "c.g"' ):
+			tweaks["out"].attributes( "/plane" )["surface"]
+
 	def testInvalidInShaderAssignment( self ) :
 
 		plane = GafferScene.Plane()