chore: add grind_trig test case (#8476)

tests/lean/run/grind_trig.lean

@@ -0,0 +1,36 @@
+set_option grind.warning false
+
+axiom R : Type
+instance : Lean.Grind.CommRing R := sorry
+
+axiom cos : R → R
+axiom sin : R → R
+axiom trig_identity : ∀ x, (cos x)^2 + (sin x)^2 = 1
+
+grind_pattern trig_identity => cos x
+grind_pattern trig_identity => sin x
+
+-- Whenever `grind` sees `cos` or `sin`, it adds `(cos x)^2 + (sin x)^2 = 1` to the blackboard.
+-- That's a polynomial, so it is sent to the Grobner basis module.
+-- And we can prove equalities modulo that relation!
+example : (cos x + sin x)^2 = 2 * cos x * sin x + 1 := by
+  grind +ring
+
+-- `grind` notices that the two arguments of `f` are equal,
+-- and hence the function applications are too.
+example (f : R → Nat) : f ((cos x + sin x)^2) = f (2 * cos x * sin x + 1) := by
+  grind +ring
+
+-- After that, we can use basic modularity conditions:
+-- this reduces to `4 * x ≠ 2 + x` for some `x : ℕ`
+example (f : R → Nat) : 4 * f ((cos x + sin x)^2) ≠ 2 + f (2 * cos x * sin x + 1) := by
+  grind +ring
+
+-- A bit of case splitting is also fine.
+-- If `max = 3`, then `f _ = 0`, and we're done.
+-- Otherwise, the previous argument applies.
+example (f : R → Nat) : max 3 (4 * f ((cos x + sin x)^2)) ≠ 2 + f (2 * cos x * sin x + 1) := by
+  grind +ring
+
+-- See https://github.com/leanprover-community/mathlib4/blob/nightly-testing/MathlibTest/grind/trig.lean
+-- for the Mathlib version of this test, using the real `ℝ`, `cos`, `sin`, and `cos_sq_and_sin_sq` declarations.