`conv`-mode `congr` leaves unsynthesized placeholder

Prerequisites

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• Check that your issue is not already filed:
  https://github.com/leanprover/lean4/issues
• Reduce the issue to a minimal, self-contained, reproducible test case.
  Avoid dependencies to Mathlib or Batteries.
• Test your test case against the latest nightly release, for example on
  https://live.lean-lang.org/#project=lean-nightly
  (You can also use the settings there to switch to “Lean nightly”)

Description

The congr tactic within conv mode leaves a stray uninstantiated metavariable, leading to a cryptic error

Context

Zulip thread

Steps to Reproduce

Run

theorem T: (fun i => if h : i < 1 then 42 else 0) = fun i => 42 := by
  conv =>
    lhs
    intro i
    congr
  sorry

Expected behavior: the sorry should succeed

Actual behavior:

don't know how to synthesize placeholder for argument 'α'
context:
⊢ Prop

on the sorry

Versions

4.14.0-rc2
4.15.0-nightly-2024-11-22

Additional Information

If using the recover sorry tactic from import Mathlib.Tactic.Recover, it turns out that there is a stray (i : Nat) → Decidable (i < 1) goal.

Impact

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Start by running the minimal reproducer from the issue against the reported Lean versions and inspect the conv-mode congr behavior. Use the reported stray (i : Nat) → Decidable (i < 1) goal as the failure criterion; done means the sorry succeeds without leaving an unsynthesized placeholder.