Commit d5773a49 authored by Simon Peyton Jones's avatar Simon Peyton Jones

Teach DmdAnal that coercions are value arguments!

The demand analyser was treating coercion args like type args,
which meant that the arguments in a strictness signature got
out of step with the arguments of a call.  Result chaos and
disaster.  Trac #10288 showed it up.

It's hard to get this bug to show up in practice because
 - functions abstracted over coercions are usually abstracted
   over *boxed* coercions
 - we don't currently unbox a boxed-coercion arg because it's
   GADT (I see how to fix this too)

But after floating, optimisation, and so on, Trac #10288 did
get a function abstracted over an unboxed coercion, and then
the -flate-dmd-anal pass went wrong.

I don't think I can come up with a test case, but I don't think
it matters too much.

Still to come
 - Fix a second bug, namely that coercion variables are wrongly
   marked as absent because DmdAnal doesn't check the the free
   variables of casts. I think this never bites in practice
   (see the follow-up commit)

 - Make GADT products work with strictness analysis
parent 4bc925a6
......@@ -164,15 +164,13 @@ dmdAnal' env dmd (App fun (Type ty))
(fun_ty, fun') = dmdAnal env dmd fun
dmdAnal' sigs dmd (App fun (Coercion co))
= (fun_ty, App fun' (Coercion co))
(fun_ty, fun') = dmdAnal sigs dmd fun
-- Lots of the other code is there to make this
-- beautiful, compositional, application rule :-)
dmdAnal' env dmd (App fun arg) -- Non-type arguments
= let -- [Type arg handled above]
dmdAnal' env dmd (App fun arg)
= -- This case handles value arguments (type args handled above)
-- Crucially, coercions /are/ handled here, because they are
-- value arguments (Trac #10288)
= let
call_dmd = mkCallDmd dmd
(fun_ty, fun') = dmdAnal env call_dmd fun
(arg_dmd, res_ty) = splitDmdTy fun_ty
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