Hi, Stephanie stumbled on this apparent inconsistency in CoreSubst, about what ought to be in the in_scope_set of a Subst. On the one hand, the file specifies #in_scope_invariant# The in-scope set contains at least those 'Id's and 'TyVar's that will be in scope /after/ applying the substitution to a term. Precisely, the in-scope set must be a superset of the free vars of the substitution range that might possibly clash with locally-bound variables in the thing being substituted in. Note that the first sentence does not actually imply the second (unless you replace “Precisely” with “In particular”). But the comment even explicitly states: Make it empty, if you know that all the free vars of the substitution are fresh, and hence can't possibly clash Looking at the code I see that lookupIdSubst indeed expects all variables in either the actual substitution or in the in_scope_set: lookupIdSubst :: SDoc -> Subst -> Id -> CoreExpr lookupIdSubst doc (Subst in_scope ids _ _) v | not (isLocalId v) = Var v | Just e <- lookupVarEnv ids v = e | Just v' <- lookupInScope in_scope v = Var v' -- Vital! See Note [Extending the Subst] | otherwise = WARN( True, text "CoreSubst.lookupIdSubst" <+> doc <+> ppr v $$ ppr in_scope) Var v Note the warning! It seems that one of these three are true: A The invariant should be the first sentence; in particular; the in_scope_set contains all the free variables that are not substituted. The rest of that comment needs to be updated to reflect that. B The invariant should be the second sentence, and the WARN is bogus, i.e. WARNs about situations that are actually ok. The rest of that comment needs to be updated, and the WARN removed. C The invariant should be the second sentence, and the WARN is still ok there because, well, it is only a warning and only appears in DEBUG builds. The rest of that comment needs to be updated, the WARN remains. Which one is it? Cheers, Joachim -- Joachim Breitner mail@joachim-breitner.de http://www.joachim-breitner.de/