Return types not inferred in presence of implicit variables of unknown type

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

These two functions throw an error:

def foo (eq: x = x) := eq
def bar (eq: x = x) := trivial

Possible workaround A: provide explicit return type annotations

def foo (eq: x = x): x = x := eq
def bar (eq: x = x): True := trivial

Possible workaround B: explicitly declare the implict variables

def foo {x: X} (eq: x = x) := eq
def bar {x: X} (eq: x = x) := trivial

Expected behavior: The types are inferred correctly, there are no errors.

Actual behavior:

type mismatch
  trivial
has type
  True : Prop
but is expected to have type
  ?m.94 eq : Sort ?u.91

Versions

4.12.0-nightly-2024-10-19
OS / platform: live.lean-lang.org

Impact

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Reproduce the two definitions in the Lean nightly environment or on live.lean-lang.org, then compare the reported type mismatch with the expected behavior and both documented workarounds. Done means the minimal examples compile without explicit return-type annotations or implicit-variable declarations.