[GHC] #10627: Regression: cabal install of numeric-prelude hangs

[GHC]

#10627: Regression: cabal install of numeric-prelude hangs
-------------------------------------+-------------------------------------
        Reporter:  George            |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  high              |               Milestone:  7.10.2
       Component:  Compiler          |                 Version:  7.10.2-rc2
      Resolution:                    |                Keywords:
Operating System:  MacOS X           |            Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash                              |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
-------------------------------------+-------------------------------------
Changes (by simonpj):

 * cc: ghc@…, george (removed)
 * os:  Unknown/Multiple => MacOS X


Comment:

 Amazing.  I couldn't resist looking at this, and it seems to be a very
 long-standing bug, introduced I think by
 {{{
 commit 30c17e7096919c55218083c8fcb98e6287552058
 Author: simonpj at microsoft.com <unknown>
 Date:   Thu Nov 25 17:23:56 2010 +0000

     Substitution should just substitute, not optimise

     This was causing Trac #4524, by optimising
          (e |> co)  to   e
     on the LHS of a rule.  Result, the template variable
     'co' wasn't bound any more.

     Now that substition doesn't optimise, it seems sensible to call
     simpleOptExpr rather than substExpr when substituting in the
     RHS of rules.  Not a big deal either way.
 }}}
 The last para says "optimise the RHS of rules when substituting", but that
 is too strict in the `IdInfo` of an `Id` if the RULE refers to the same
 `Id`.

 I don't know how this ever worked.  Patch coming.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10627#comment:16>
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