code works in 7.0 but hangs in 7.2/7.4 due to changes in type checker (most likely)

I have a function with the lovely type signature:

homePage :: (Happstack m, 
             MonadRoute m,
             MonadUser m, 
             HasAppConf m, 
             ToMessage (HSX.XML m), 
             MonadRender m, 
             Ontology.MkURL (URL m),
             EmbedAsAttr m (Attr String (URL m))) =>
            m Response

The code works fine in 7.0 but hangs (at runtime) under 7.2 and 7.4rc1.

If I change that type signature to the more specific type:

homePage :: AppPart' Response

Then the code starts working again.

This bug reminds me a lot of these previously closed bugs:

#4809 (closed) #3731 (closed)

I will follow up with an isolated test case in the next couple days. It will likely take some time to tease out the relevant parts of the code into something more manageable.

Trac metadata

Trac field	Value
Version	7.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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