error in 7.10.1 ?

[Sergei Meshveliani]

Dear GHC developers,

The confirmation token letter from the bug tracker travels too long,  I
do not know, why.
Meanwhile I cannot report there.
So, I report it in the below way: 


http://www.botik.ru/pub/local/Mechveliani/ghcQuest/7.10.1-errReport-may22-2015.zip


Make it, please, with  ghc-7.10.1   as it is written there in
install.txt,
including making   demotest/Main.
Then
         ./Main

breaks in the middle reporting a DoCon application error

----------------------------------------------------------------------------
Factorization in  K1[x],  K1  a generic finite extension of a prime
field K = Z/(p) .. 
factor f,  
 f =  (x^10 + x^7 + x^5 + x^3 + x^2 + (2)*x+ (2))  <-  K[x] =
((Z/<5>)["t"]/<(t^3 + 4*t + 3)>)["x"]
 Domain term of K must contain a Finite field description.
---------------------------------------------------------------------------

I obtain this for   ghc-7.10.1  made from source by  ghc-7.8.2  on
Debian Linux,
x-86, 64 bit. 

The story is as follows.

docon-2.12
      http://www.botik.ru/pub/local/Mechveliani/docon/2.12/

has been tested under   ghc-7.8.2,   and it works.

Now I am trying to port it to  ghc-7.10.1.  
This needs inserting certain pragmas for  OVERLAPPING/OVERLAPPABLE into
some instance declarations.
So I call this port 
                           docon-2.12.1.

7.10.1-errReport-may22-2015.zip   is the error report  of  docon-2.12.1
to  ghc-7.10.1.

This application has the OVERLAPPABLE pragma set to one instance and the
OVERLAPPING pragma set to many instances. 
GHC itself warns of overlaps. Then I 
1) set the pragma to the reported instance (pair),
2) recompile the corresponding module
3) rerun
    make configure;  make build.

This process has finished in `making'  the application.
Then,
           > cd demotest
           > ghc $doconCpOpt -O --make -rtsopts Main
           > ./Main

produces the above error instead of the test success report.

I suspect that the reason is in treating the instance overlaps
(but there may be another source).

Comparing   docon-2.12  + ghc-7.8.2 
to                  7.10.1-errReport-may22-201 + ghc-7.10.1,

can you, please, find:  how to fix the latter version?
Is this a bug in ghc-7.10.1 ?

Regards,

--------
Sergei










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