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

Assisted-by: DeepSeek V4.1 Flash

CHANGELOG.md

@@ -45,6 +45,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   interleaves the endpoints of unrelated edges.
 - `IntegrateFunction` names itself in its errors; the adaptive
   quadrature no longer reports under a name absent from the surface.
+- `MinimiseDifferentialEvolution` refuses a non-finite bound instead
+  of drawing a NaN population and returning a NaN point with no
+  error.
 
 ## [1.0.0] - 2026-09-03
 

optim/devolution.go

@@ -58,8 +58,12 @@ func MinimiseDifferentialEvolution(f func(*core.Array) (float64, error),
 		return nil, 0, base.Errf("%s: complex bounds are not supported", name)
 	}
 	for i := range n {
-		if !(upper.FloatAt(i) > lower.FloatAt(i)) {
-			return nil, 0, base.Errf("%s: bound %d runs from %g to %g", name, i, lower.FloatAt(i), upper.FloatAt(i))
+		lo, up := lower.FloatAt(i), upper.FloatAt(i)
+		// An infinite side has no uniform draw: the population would be
+		// born NaN and the best point would come back NaN with no error,
+		// so a non-finite bound is degenerate exactly as a crossed one is.
+		if math.IsNaN(lo) || math.IsNaN(up) || math.IsInf(lo, 0) || math.IsInf(up, 0) || !(up > lo) {
+			return nil, 0, base.Errf("%s: bound %d runs from %g to %g", name, i, lo, up)
 		}
 	}
 	pop := opts.Population

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))
+		}
+	}
+}