Re: [GHC] #12885: "too many iterations" causes constraint solving issue.

#12885: "too many iterations" causes constraint solving issue. -------------------------------------+------------------------------------- Reporter: judahj | Owner: Type: bug | Status: merge Priority: normal | Milestone: Component: Compiler | Version: 8.0.2-rc1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * status: new => merge Comment: Aha! Happily, this patch (committed last week) fixes it {{{ commit 0476a64e70c91b326b53db2fc55adbbaa8e5c270 Author: Simon Peyton Jones Date: Tue Oct 25 15:22:17 2016 +0100 Fix a bug in mk_superclasses_of This bug meant that we were less eager about expanding tuple superclasses than we should have been; i.e. we stopped too soon. That's not fatal, beause we expand more superclasses later, but it's less efficient. }}} It's fine in HEAD. I suggest we just merge the patch across for 8.0.3. Here's a one-module test case {{{ {-# LANGUAGE TypeFamilies, ConstraintKinds #-} module Foo where import GHC.Exts f :: F [a] => a -> Bool f x = x == x type family F a :: Constraint type instance F [a] = (Show a, (Show a, (Show a, (Show a, (Show a, Show a, (Show a, (Show a, (Show a, (Show a, Eq a))))))))) }}} The Given `F [a]` expands to the nested tuple, and the bug is that we were only expanding one level of the tuple at a time. I'll add this test to HEAD, for what it's worth. Thanks for identifying this! Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12885#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler