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 @~ (<s_Xpw a_Xpz>_N :: s_Xpw a_Xpz ~# s_Xpw a_Xpz) @~ (<s_Xpw b_Xpy>_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: <s_Xpw a_Xpz>_N, CO: <s_Xpw b_Xpy>_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
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