Hello Bartosz, The principle between uniqAway is described in the "Secrets of the GHC Inliner" paper http://research.microsoft.com/en-us/um/people/simonpj/Papers/inlining/ I doubt there's a bug in uniqAway; it's more likely the in scope set is not correct. Edward Excerpts from Bartosz Nitka's message of 2016-01-05 09:39:54 -0800:
Hi,
I'm running into core lint issues [1] with my determinism patch [2] and they appear to come down to uniqAway coming up with a Unique that's already used in the expression. That creates a collision, making next substitution substitute more than it needs.
I have 2 questions:
1. When is it safe to use uniqAway?
2. Is my conclusion reasonable given this trace: (full trace here [3])?
I. We start out with (line 123):
CallStack (from ImplicitParams): pprSTrace, called at compiler/coreSyn/CoreLint.hs:659:12 in ghc:CoreLint e: EqConstr @ (s_Xpw a_Xpz) @ (s_Xpw b_Xpy) @ s_Xpw @ b_Xpy @ a_Xpz @~ (
_N :: s_Xpw a_Xpz ~# s_Xpw a_Xpz) @~ ( _N :: s_Xpw b_Xpy ~# s_Xpw b_Xpy) dt_aq3 fun: EqConstr args: [TYPE: s_Xpw a_Xpz, TYPE: s_Xpw b_Xpy, TYPE: s_Xpw, TYPE: b_Xpy, TYPE: a_Xpz, CO: _N, CO: _N, dt_aq3] fun_ty: forall a_apr b_aps (s_Xpw :: * -> *) b_Xpy a_Xpz. (a_apr ~# s_Xpw a_Xpz, b_aps ~# s_Xpw b_Xpy) => EqTypes a_Xpz b_Xpy -> EqTypes a_apr b_aps II. Then we create s_Xpz with uniqAway (line 499) which has the same unique as a_Xpz above
CallStack (from ImplicitParams): pprSTrace, called at compiler/types/TyCoRep.hs:1947:5 in ghc:TyCoRep old_var: s_Xpy new_var: s_Xpz
III. Which causes trouble when we want to substitute s_Xpw for s_Xpz and we substitute a_Xpz as well (line 733):
CallStack (from ImplicitParams): pprSTrace, called at compiler/coreSyn/CoreLint.hs:813:11 in ghc:CoreLint substTyWith [tv] [arg_ty] body_ty forall b_XpB a_XpD. (s_Xpw s_Xpw ~# s_Xpw a_XpD, s_Xpw b_Xpy ~# s_Xpw b_XpB) => EqTypes a_XpD b_XpB -> EqTypes (s_Xpw s_Xpw) (s_Xpw b_Xpy)
The uniqAway comes from substTyVarBndrCallback which blames to nokinds patch.
Thanks, Bartosz [1] https://phabricator.haskell.org/P77 [2] https://phabricator.haskell.org/P76 [3] https://phabricator.haskell.org/P78