[GHC] #14643: Partial type signatures in class constraints behave unexpectedly

[GHC]

#14643: Partial type signatures in class constraints behave unexpectedly
-------------------------------------+-------------------------------------
        Reporter:  mnislaih          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"1577908f2a9db0fcf6f749d40dd75481015f5497/ghc" 1577908/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="1577908f2a9db0fcf6f749d40dd75481015f5497"
 Fix two more bugs in partial signatures

 These were shown up by Trac #14643

 Bug 1: if we had a single partial signature for
 two functions
    f, g :: forall a. _ -> a
 then we made two different SigTvs but with the sane Name.
 This was jolly confusing and ultimately led to deeply bogus
 results with Any's appearing in the resulting program. Yikes.
 Fix: clone the quantified variables in TcSigs.tcInstSig (as
 indeed its name suggests).

 Bug 2: we were not eliminating duplicate/superclass constraints
 in the partial signatures of a mutually recursive group.

 Easy to fix: we are already doing dup/superclass elim in
 TcSimplify.decideQuantification.  So we move the partial-sig
 constraints there too.
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14643#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler