`grind` fails to prove conjunction of two things it can prove

[TwoFX]

Prerequisites

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Description

See the following snippet:

example {a b c d : Int} (f : Int → Nat → Int) (h₁ : a = 0) (h₁' : ∀ k, 0 ≤ f c k) (h₂ : b = 0) (h₂' : ∀ k, 0 ≤ f d k) :
    a + b = 0 := by
  grind -- works

example {a b c d : Int} (f : Int → Nat → Int) (h₁ : a = 0) (h₁' : ∀ k, 0 ≤ f c k) (h₂ : b = 0) (h₂' : ∀ k, 0 ≤ f d k) :
    ∀ k, 0 ≤ f c k + f d k := by
  grind -- works

example {a b c d : Int} (f : Int → Nat → Int) (h₁ : a = 0) (h₁' : ∀ k, 0 ≤ f c k) (h₂ : b = 0) (h₂' : ∀ k, 0 ≤ f d k) :
    a + b = 0 ∧ ∀ k, 0 ≤ f c k + f d k := by
  rw [h₁]
  grind -- works

example {a b c d : Int} (f : Int → Nat → Int) (h₁ : a = 0) (h₁' : ∀ k, 0 ≤ f c k) (h₂ : b = 0) (h₂' : ∀ k, 0 ≤ f d k) :
    a + b = 0 ∧ ∀ k, 0 ≤ f c k + f d k :=
  ⟨by grind, by grind⟩ -- works

example {a b c d : Int} (f : Int → Nat → Int) (h₁ : a = 0) (h₁' : ∀ k, 0 ≤ f c k) (h₂ : b = 0) (h₂' : ∀ k, 0 ≤ f d k) :
    a + b = 0 ∧ ∀ k, 0 ≤ f c k + f d k := by
  grind -- fails

The diagnostics don't show any reached thresholds.

Expected behavior: grind solves last goal

Actual behavior: grind fails

Versions

4.28.0 and 4.30.0-nightly-2026-02-18 on live.lean-lang.org

Impact

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[TwoFX]

Claude suggests the following one-line fix, which does fix the issue, but naturally I have no idea whether it's a good fix.

diff --git a/src/Lean/Meta/Tactic/Grind/Finish.lean b/src/Lean/Meta/Tactic/Grind/Finish.lean
index 6bae01e1c2..c3006db620 100644
--- a/src/Lean/Meta/Tactic/Grind/Finish.lean
+++ b/src/Lean/Meta/Tactic/Grind/Finish.lean
@@ -14,7 +14,7 @@ public abbrev maxIterationsDefault := 10000 -- **TODO**: Add option
 
 public def mkFinish (maxIterations : Nat := maxIterationsDefault) : IO Action := do
   let solvers ← Solvers.mkAction
-  let step : Action := solvers <|> instantiate <|> splitNext <|> mbtc
+  let step : Action := (assertNext >> assertAll) <|> solvers <|> instantiate <|> splitNext <|> mbtc
   return checkTactic (warnOnly := true) >> intros 0 >> assertAll >> step.loop maxIterations
 
 end Lean.Meta.Grind.Action