[GHC] #14434: GHC 8.2.1 picks wrong instances with -O in a presence of an OVERLAPPABLE instance

[GHC]

#14434: GHC 8.2.1 picks wrong instances with -O in a presence of an OVERLAPPABLE
instance
-------------------------------------+-------------------------------------
        Reporter:  dredozubov        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.3
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:  x86_64
 Type of failure:  Incorrect result  |  (amd64)
  at runtime                         |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by dredozubov:

Old description:

> We experienced code breakage while moving to 8.2.1 - in some
> circumstances it picks wrong instances if an instance in the lines of
> `instance {-# OVERLAPPABLE #-} C a ` is defined. @s9gf4ult was really
> nice to create a repo demonstrating an issue, you can look at the repo
> here: [https://github.com/s9gf4ult/isogen-minimal-example]
>
> Running `chech.sh` will trigger three builds:
>

> {{{
> GHC 8.0.2, -O, succeeds
> GHC 8.2.1, -O0, succeeds
> GHC 8.2.1, -O, fails
> }}}
>

>
> If you'll check out to either of fix- branches:
>

> {{{
> fix-move-to_string
> fix-no-catchall-instance
> fix-replace-to_string
> fix-add-irrelevant-instance
> }}}
>
> then the last test should succeed also. Those branches contain changes
> over master which fixes the bug in different ways.

New description:

 We experienced code breakage while moving to 8.2.1 - in some circumstances
 it picks wrong instances if an instance in the lines of `instance {-#
 OVERLAPPABLE #-} C a ` is defined. @s9gf4ult was really nice to create a
 repo demonstrating an issue, you can look at the repo here:
 [https://github.com/s9gf4ult/isogen-minimal-example]

 Running `chech.sh` will trigger three builds:


 {{{
 GHC 8.0.2, -O, succeeds
 GHC 8.2.1, -O0, succeeds
 GHC 8.2.1, -O, fails
 }}}



 If you'll check out to either of fix- branches:


 {{{
 fix-move-to_string
 fix-no-catchall-instance
 fix-replace-to_string
 fix-add-irrelevant-instance
 }}}

 then the last test should succeed also. Those branches contain changes
 over master which fix the application behavior in different ways.

--

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14434#comment:3>
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