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eric-wieser
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Jul 16, 2023
Mathlib/Util/Qq.lean
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| /-- `Lean.toExpr` with Qq support. -/ | ||
| def Lean.ToExpr.mkQ.{u} {α : Type u} [Lean.ToLevel.{u}] | ||
| (toTypeExprQ : Q(Type $(Lean.toLevel.{u}))) (toExprQ : α → Q($toTypeExprQ)) : | ||
| ToExpr α where | ||
| toTypeExpr := toTypeExprQ | ||
| toExpr x := toExprQ x |
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This definition seems to get confused with universe variables if I actually try to use it.
eric-wieser
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Jul 16, 2023
Mathlib/Data/Fin/Basic.lean
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| instance toExpr (n : ℕ) : Lean.ToExpr (Fin n) where | ||
| toTypeExpr := q(Fin $n) | ||
| toExpr := match n with | ||
| instance toExpr (n : ℕ) : Lean.ToExpr (Fin n) := Lean.ToExpr.mkQ.{0} |
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Without the {0} this gives
type mismatch
Fin «$n»
has type
Type : Type 1
but is expected to have type
Type 127 : Type 128
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Yes, this is the ToLevel issue I've alluded to on Zulip. Depending on the order the instances are synthesized, we can end up searching for an instance of ToLevel.{?u}. And since the succ instances has higher priority than the zero instance, we get ToLevel.{127} as a result. If we'd swap the instance priorities, we'd get ToLevel.{0}. Of course, neither choice is desirable.
eric-wieser
commented
Jul 17, 2023
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This PR/issue depends on:
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Coming here from triaging old PRs: is this PR still relevant? (I vaguely remember some material around ToExpr being upstreamed to core - that might have been something else, though.) |
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ToExpr#6699 (to appease the linter)