pp.all cannot round-trip applied lambdas with implicit binders

Prerequisites

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• 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
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Description

Even pp.all true seems to not be enough to print terms of the form Expr.app `(fun {x} => y) z.

Context

Zulip thread

Steps to Reproduce

Run

import Lean

open Lean

elab "thats_illegal" : term =>
  return Expr.app
    (Expr.lam `n (.const `Nat []) (.bvar 0) .implicit)
    (Expr.const `Nat.zero [])

set_option pp.all true

/-- info: (@fun {n : Nat} => n) Nat.zero : Nat -/
#guard_msgs in #check thats_illegal

-- copied from the output, but doesn't work!
#check ((@fun {n : Nat} => n) Nat.zero : Nat)

-- fixed by hand, but not the same expression any more
#check (@id _ (fun {n : Nat} => n) Nat.zero : Nat)

Expected behavior: The output of #check with pp.all true should round-trip

Actual behavior: The resulting expression has a type error

Versions

4.14.0-rc2

Additional Information

[Additional information, configuration or data that might be necessary to reproduce the issue]

Impact

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Start with the minimal Lean reproduction using pp.all true, Expr.app, and the #check commands. Compare the printed applied lambda with the hand-fixed expression and trace how pretty-printed implicit binders are parsed. Done means the #check output round-trips without a type error while preserving the intended expression.