`grind [function_name]` does not work on typeclass functions

Prerequisites

• 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

class Test (α: Type) where
  pred: α → Prop

instance: Test Unit where
  pred _ := True

example: Test.pred () := by
  fail_if_success grind [Test.pred]
  grind [Test.pred, instTestUnit]

Steps to Reproduce

Expected behavior: the first grind succeeds

Actual behavior: it does not

Versions

"4.28.0-nightly-2025-12-16"
"4.26.0"

Impact

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Start by reproducing the minimal example with grind [Test.pred] and grind [Test.pred, instTestUnit], then trace how grind processes explicit arguments for the typeclass projection Test.pred. Done means the first command succeeds without needing instTestUnit, with the reproduction covered by an appropriate regression test.