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WIP: DsExpr: Make result of mfix in rec do more lazy

Ellie Hermaszewska requested to merge expipiplus1/ghc:strict-dorec into master

With -XStrict on this result is matched strictly which is probably not desired. There doesn't seem to be a way for the programmer to make this match irrefutable when XStrict is on.

Alternatives may be looking at any lazyness annotations on the bound variables and "bubbling" those up to the tuple match.

I'm not 100% certain about the second LazyPat being used here. Although it enables the mdorun007 test to pass, I suspect that this may be due to some fortunate interaction with the strictness analyzer rather than this being the proper fix.

Edited by Ben Gamari

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