leanprover / leanprover/lean4

Syntax match strange error

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bug P-medium
Dominant language
Lean
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Merged PRs (30d)
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Description

Prerequisites
Description

Pattern matching on syntax can lead to unexpected errors as in:

import Lean

open Lean Meta

def foo (t : Term) : Name :=
  match t with -- Failed to infer binder type
  | `($x:ident > 0) =>  .mkSimple <| x.getId.toString ++ "_pos"
  | `($x:ident > $y:ident) => .mkSimple <| x.getId.toString ++ "_gt_" ++ y.getId.toString
  | `($x:ident > $_) =>  x.getId
  | _ => `Bas
Context

See variations on https://leanprover.zulipchat.com/#narrow/channel/270676-lean4/topic/Weird.20errors.20in.20quotation.20match

Steps to Reproduce

The above code is enough.

Expected behavior:

This definition should work

Actual behavior:

There is a “Failed to infer binder type” error on the match line.

Versions

4.27.0-rc1

Impact

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Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the self-contained foo definition in the issue and reproduce the error against Lean 4.27.0-rc1 and the latest nightly. Compare the quotation patterns in the match arms, then verify that the definition elaborates without “Failed to infer binder type” while preserving the expected Name results.

Written by the indexing model from the issue text.

Assessment

Domain
compilers
Issue type
Bug
Difficulty
3/5
Estimated time
1-2 days
Activity status
Stale
Clarity
Clearly specified
Newbie friendliness
35/100

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