[GHC] #12415: Fancy BinIface encoding for tuples is broken for constraint tuples

[GHC]

#12415: Fancy BinIface encoding for tuples is broken for constraint tuples
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        Reporter:  bgamari           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12357            |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by bgamari:

@@ -26,1 +26,2 @@
- to identify them for special handling during serialization.
+ to identify them for special handling during serialization, when we only
+ have the `Name` to look at.

New description:

 We have a special way of encoding tuple-related names in the interface
 file symbol table. See `Note [Symbol table representation of names]`.

 However, it was broken for constraint tuples by
 ffc21506894c7887d3620423aaf86bc6113a1071. Namely, `putName` now panics
 when given a constraint tuple,
 {{{#!hs
 putTupleName_ :: BinHandle -> TyCon -> TupleSort -> Word32 -> IO ()
 putTupleName_ bh tc tup_sort thing_tag
   = -- ASSERT(arity < 2^(30 :: Int))
     put_ bh (0x80000000 .|. (sort_tag `shiftL` 28) .|. (thing_tag `shiftL`
 26) .|. arity)
   where
     (sort_tag, arity) = case tup_sort of
       BoxedTuple      -> (0, fromIntegral (tyConArity tc))
       UnboxedTuple    -> (1, fromIntegral (tyConArity tc `div` 2))
         -- See Note [Unboxed tuple RuntimeRep vars] in TyCon
       ConstraintTuple -> pprPanic "putTupleName:ConstraintTuple" (ppr tc)
 }}}
 While this currently doesn't break anything, this is only because the
 clever encoding isn't used for constraint tuples.

 I believe the problem is very similar to the one which prevented us from
 encoding tuple type representations in #12357 (see
 ticket:12357#comment:31). By unwiring constraint tuples it becomes harder
 to identify them for special handling during serialization, when we only
 have the `Name` to look at.

--

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12415#comment:2>
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