@@ -83,6 +83,8 @@ class UnionType implements CompoundType
 	/** @var list<Type>|null */
 	private ?array $finiteTypes = null;
 
+	private ?TrinaryLogic $isNull = null;
+
 	/**
 	 * @api
 	 * @param list<Type> $types
@@ -1163,7 +1165,7 @@ public function isSmallerThanOrEqual(Type $otherType, PhpVersion $phpVersion): T
 
 	public function isNull(): TrinaryLogic
 	{
-		return $this->notBenevolentUnionResults(static fn (Type $type): TrinaryLogic => $type->isNull());
+		return $this->isNull ??= $this->notBenevolentUnionResults(static fn (Type $type): TrinaryLogic => $type->isNull());
 	}
 
 	public function isConstantValue(): TrinaryLogic
Original file line number	Diff line number	Diff line change
`@@ -83,6 +83,8 @@ class UnionType implements CompoundType`
`83`	`83`	`/** @var list<Type>\|null */`
`84`	`84`	`private ?array $finiteTypes = null;`
`85`	`85`
	`86`	`+ private ?TrinaryLogic $isNull = null;`
	`87`	`+`
`86`	`88`	`/**`
`87`	`89`	`* @api`
`88`	`90`	`* @param list<Type> $types`
`@@ -1163,7 +1165,7 @@ public function isSmallerThanOrEqual(Type $otherType, PhpVersion $phpVersion): T`
`1163`	`1165`
`1164`	`1166`	`public function isNull(): TrinaryLogic`
`1165`	`1167`	`{`
`1166`		`- return $this->notBenevolentUnionResults(static fn (Type $type): TrinaryLogic => $type->isNull());`
	`1168`	`+ return $this->isNull ??= $this->notBenevolentUnionResults(static fn (Type $type): TrinaryLogic => $type->isNull());`
`1167`	`1169`	`}`
`1168`	`1170`
`1169`	`1171`	`public function isConstantValue(): TrinaryLogic`