I can’t reproduce this. With the enclosed module and HEAD, I get the warning; but when I add –fspec-constr-count=5, the warning goes away and I get the specialised
rules.
Could Cabal not be passing on the flag or something?
I see -fspec-constr-count=5 in the command line being passed to ghc when I run "cabal build -v", so I don't think that's to blame?