We were mislabelling some foreign imports as IsData rather than
IsFunction, because the type was forall a. FunPtr (... a ...).

Eventually this gave rise to these obscure error when compiling
unregisterised:

libraries/base/Foreign/Marshal/Alloc.hc:473:0:
     warning: built-in function ‘free’ declared as non-function

libraries/base/Foreign/Marshal/Alloc.hc:1004:0:
     error: ‘free’ redeclared as different kind of symbol