fix(cmp): handle NaN values properly

.golangci.yml

@@ -122,3 +122,9 @@ issues:
       text: "comment on exported const .+ should be of the form"
     - linters: [revive]
       text: "exported const .+ should have comment"
+
+    # False positive.
+    - path: internal/cmp
+      linters: [gocritic]
+      source: "return x != x"
+      text: "dupSubExpr"

internal/cmp/cmp.go

@@ -3,14 +3,34 @@ package cmp
 
 import "golang.org/x/exp/constraints"
 
-// Compare is a comparator.
-func Compare[T constraints.Ordered](a, b T) int {
-	switch {
-	case a < b:
-		return -1
-	case a > b:
-		return 1
-	default:
+// Ordered is a type alias to [constraints.Ordered].
+type Ordered = constraints.Ordered
+
+// Compare returns
+//
+//	-1 if x is less than y,
+//	 0 if x equals y,
+//	+1 if x is greater than y.
+//
+// For floating-point types, a NaN is considered less than any non-NaN,
+// a NaN is considered equal to a NaN, and -0.0 is equal to 0.0.
+func Compare[T Ordered](x, y T) int {
+	xNaN := isNaN(x)
+	yNaN := isNaN(y)
+	if xNaN && yNaN {
 		return 0
 	}
+	if xNaN || x < y {
+		return -1
+	}
+	if yNaN || x > y {
+		return +1
+	}
+	return 0
+}
+
+// isNaN reports whether x is a NaN without requiring the math package.
+// This will always return false if T is not floating-point.
+func isNaN[T Ordered](x T) bool {
+	return x != x
 }