It was done intentionally, to avoid having the zipWith semantics depend on which, if either, of the venerable foldr2 RULES fired. But I'd personally be very happy to just nix the shady rule (the other is fine), and accept the performance penalty.
On Fri, 2 Jan 2015, Roman Cheplyaka wrote:
I don't think it does. The first pattern (a:as) will be refuted, and the
second (b:bs) won't try to match at all.
right - stupid me
Then, the new behavior of zipWith is really a bug. I added a ticket:
https://ghc.haskell.org/trac/ghc/ticket/9949
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