fix(optim): refuse non-finite bounds in differential evolution

Assisted-by: DeepSeek V4.1 Flash

optim/devolution_test.go

@@ -112,3 +112,25 @@ func TestDEErrors(t *testing.T) {
 		t.Error("NaN objective accepted")
 	}
 }
+
+// TestDERejectsInfiniteBounds pins that an infinite side is refused: the
+// sampler draws uniformly inside the box, so an open side would populate
+// it with NaN and hand back a NaN point with a nil error.
+func TestDERejectsInfiniteBounds(t *testing.T) {
+	zero, _ := core.FromFloats([]float64{0}, 1)
+	one, _ := core.FromFloats([]float64{1}, 1)
+	constF := func(a *core.Array) (float64, error) { return 1, nil }
+	for _, tc := range []struct {
+		name   string
+		lo, hi *core.Array
+	}{
+		{"both sides open", mustBound(math.Inf(-1)), mustBound(math.Inf(1))},
+		{"upper open", zero, mustBound(math.Inf(1))},
+		{"lower open", mustBound(math.Inf(-1)), one},
+	} {
+		x, _, err := MinimiseDifferentialEvolution(constF, tc.lo, tc.hi, DifferentialEvolutionOptions{Generations: 5})
+		if err == nil {
+			t.Errorf("%s: infinite bound accepted, returned x=%g", tc.name, x.FloatAt(0))
+		}
+	}
+}