[commit: ghc] master: Make the demand on a binder compatible with type (fixes Trac #8569) (4b355cd)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/4b355cd21a190e3d2c2d3a830ba2337d1c442dfe/ghc

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commit 4b355cd21a190e3d2c2d3a830ba2337d1c442dfe
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Thu Mar 6 11:31:47 2014 +0000

    Make the demand on a binder compatible with type (fixes Trac #8569)
    
    Because of GADTs and casts we were getting binders whose
    demand annotation was more deeply nested than made sense
    for its type.
    
    See Note [Trimming a demand to a type], in Demand.lhs,
    which I reproduce here:
    
       Note [Trimming a demand to a type]
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       Consider this:
    
         f :: a -> Bool
         f x = case ... of
                 A g1 -> case (x |> g1) of (p,q) -> ...
                 B    -> error "urk"
    
       where A,B are the constructors of a GADT.  We'll get a U(U,U) demand
       on x from the A branch, but that's a stupid demand for x itself, which
       has type 'a'. Indeed we get ASSERTs going off (notably in
       splitUseProdDmd, Trac #8569).
    
       Bottom line: we really don't want to have a binder whose demand is more
       deeply-nested than its type.  There are various ways to tackle this.
       When processing (x |> g1), we could "trim" the incoming demand U(U,U)
       to match x's type.  But I'm currently doing so just at the moment when
       we pin a demand on a binder, in DmdAnal.findBndrDmd.


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4b355cd21a190e3d2c2d3a830ba2337d1c442dfe
 compiler/basicTypes/Demand.lhs |   62 +++++++++++++++++++++++++++++++++++++-
 compiler/stranal/DmdAnal.lhs   |   64 +++++++++++++++++++++-------------------
 compiler/stranal/WwLib.lhs     |   31 ++++++++++++++++++-
 3 files changed, 124 insertions(+), 33 deletions(-)

Diff suppressed because of size. To see it, use:

    git diff-tree --root --patch-with-stat --no-color --find-copies-harder --ignore-space-at-eol --cc 4b355cd21a190e3d2c2d3a830ba2337d1c442dfe