#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: | -------------------------------------+------------------------------------- Changes (by bgamari): * priority: normal => high * milestone: => 8.2.3 Comment: Hmm, this looks pretty bad. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14434#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#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 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#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: | -------------------------------------+------------------------------------- Comment (by Simon Peyton Jones ): In [changeset:"30058b0e45e920319916be999de9c4d77da136e7/ghc" 30058b0e/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="30058b0e45e920319916be999de9c4d77da136e7" Fix another dark corner in the shortcut solver The shortcut solver for type classes (Trac #12791) was eagerly solving a constaint from an OVERLAPPABLE instance. It happened to be the only one in scope, so it was unique, but since it's specfically flagged as overlappable it's really a bad idea to solve using it, rather than using the Given dictionary. This led to Trac #14434, a nasty and hard to identify bug. }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14434#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#14434: GHC 8.2.1 picks wrong instances with -O in a presence of an OVERLAPPABLE instance -------------------------------------+------------------------------------- Reporter: dredozubov | Owner: (none) Type: bug | Status: merge Priority: high | Milestone: 8.2.3 Component: Compiler | Version: 8.2.1 Resolution: | Keywords: Operating System: Linux | Architecture: x86_64 | (amd64) Type of failure: Incorrect result | Test Case: at runtime | typecheck/should_compile/T14434 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by dredozubov): Thanks for the fix! Unfortunately I was unable to build our application with ghc HEAD due to broken dependencies. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14434#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler