unknown free variable since v4.8.0

pandaman64 commented 4 weeks ago

Prerequisites

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Description

Hi. I updated my project to v4.8.0 from v4.6.0 and started to get an unknown free variable: _kernel_fresh.19 error. It seems the nightly also has the issue. Here is the MWE:

structure NFA where
  nodes : Array Nat

def get (nfa : NFA) (i : Nat) (h : i < nfa.nodes.size) : Nat :=
  nfa.nodes[i]

-- "unknown free variable: _kernel_fresh.19" at instance
instance : GetElem NFA Nat Nat (fun nfa i => i < nfa.nodes.size) where
  getElem := get

Spelling out all arguments of getElem like getElem nfa i h := get nfa i h seems to fix the issue though.

Context

Zulip discussion: https://leanprover.zulipchat.com/#narrow/stream/270676-lean4/topic/unknown.20free.20variable.20in.20v4.2E8.2E0/near/443754071

Original code that could be compiled with v4.6.0 can be found at: https://github.com/pandaman64/lean-regex/blob/64d23c5d3ed30eff3267e8dd6280577ad7f3b321/Regex/NFA/Basic.lean#L124-L125

Steps to Reproduce

Create a project with leanprover/lean4:v4.8.0 and paste the code above.

Expected behavior: the instance definition should compile without an error.

Actual behavior: the instance definition fails with unknown free variable: _kernel_fresh.19.

Versions

4.8.0

I'm using Windows 11, but the issue reproduced with the web editor.

Impact

Add :+1: to issues you consider important. If others are impacted by this issue, please ask them to add :+1: to it.

llllvvuu commented 4 weeks ago

Looks like this is triggered by the new GetElem structure fields added in https://github.com/leanprover/lean4/commit/0963f3476c74312ba95ea67a786c203ed19e48ef.

Inlining and minimizing:

def get (_ : Nat) : Nat := Nat.zero

class GH4418 (idx : Type v) where
  getElem (i : idx) : Nat

  getElem? (i : idx) : Option Nat :=
    if True then Option.some (getElem i) else @Option.none Nat

  getElem?2 (i : idx) (_ : Nat) : Option Nat :=
    if True then Option.some (getElem i) else @Option.none Nat

-- "unknown free variable: _kernel_fresh.19" at instance
instance : GH4418 Nat where
  getElem := get

Note that this while this also triggers the issue:

  getElem? (i : idx) : List Nat :=
    if True then [getElem i] else []

  getElem?2 (i : idx) (_ : Nat) : List Nat :=
    if True then [getElem i] else []

this (a mix of the two) does not:

  getElem? (i : idx) : List Nat :=
    if True then [getElem i] else []

  getElem?2 (i : idx) (_ : Nat) : Option Nat :=
    if True then Option.some (getElem i) else @Option.none Nat

i.e. getElem? and getElem2? have to be similar

leodemoura commented 4 weeks ago

This is a bug in the common-case-eliminator in the old code generator.

leanprover / lean4