Unhelpful error when inferred type of parameter depends on succeeding parameter

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
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Description

When the expected type of a parameter depends on another parameter that comes after it, this understandably leads to a type error. But the error message is his very misleading, especially when using auto-implicits.

Steps to Reproduce

Run this:

def f (n : Nat) (xs : Vector Nat n) := xs = xs

/--
error: Application type mismatch: The argument
  xs
has type
  ?m.1
but is expected to have type
  Vector Nat n
in the application
  f n xs
-/
#guard_msgs in
example (n : Nat) (h : f n xs) : Unit := ()

Expected behavior:

Lean detects that the type of xs depends on n and clearly explains to the user that xs must be declared after n. If xs is an auto-implicit, Lean tells the user to declare xs by hand because auto-implicits are always placed in front of all other parameters.

Deluxe version: In addition to improved error messages, Lean automatically places auto-implicits after their type dependencies instead of always at front position.

Actual behavior:

Lean produces an error message that is not helpful. The error message does not explain why Vector Nat n cannot be assigned to the metavariable ?m.1, and it is impossible to infer from the error message that n is not in the environment of the metavariable.

Moreover, no inlay hints for auto-implicits are shown in the presence of this error, making it even harder to figure out that the error has to do with the placement of the auto-implicits.

Versions

Lean 4.24.0-nightly-2025-08-21
Target: x86_64-unknown-linux-gnu

Impact

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Start by running the minimal reproducer against the specified Lean nightly release or live. Trace how the dependent parameter and auto-implicit are processed, then verify that the diagnostic explains the dependency and placement problem and that inlay hints remain available in this error case.