Unrelated instance foils optimizer

I have a type (:.) a b that I use for vectors, so a 4d vector is a:.a:.a:.a:.(). This is just a linked list, so I also have a packed representation for Doubles, Vec4D and a class that relates the two representations.

The Storable instances for (:.) are inductive: if a and (b:.c) are Storable then so is (a:.b:.c). So long as the source is a peek and the destination is a poke, the method calls optimize neatly and no (:.) values are allocated.

Here's the weird part: If I use the Storable methods of (:.) and the pack/unpack methods to extend a Storable instance to Vec4D, then the optimization of the (:.) Storable methods fails. The Vec4D instances are not used at all, and yet they affect optimization of the (:.) instances.

This is probably related to #2416 (closed). If I move the Storable instance for (:.) into Main.hs, then all optimization fails regardless of any instance for Vec4D. If I move the Storable instance for Vec4D into a different module than the instance for (:.), then optimization works fine.

The example is in two files. I tried to merge them into one but that's when I discovered #2416 (closed).

Compiled with -O2.

Trac metadata

Trac field	Value
Version	6.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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