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Fix a unboxed-binding bug in SpecConstr
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[HEAD only]
This fixes a rather obscure bug in the constructor
specialiser discovered by Ralf Hinze. It was
generating a specialised version of the function
with no arguments --- and the function returned an
unboxed type.
Solution: same as for worker-wrapper; add a dummy
argument.
Several files are affected because I added
CoreUtils.mkPiTypes, as a useful helper function.