@@ -81,9 +81,12 @@ public static int slotOf(long tagId) {
     return (int) (tagId >>> SLOT_SHIFT) & 0xFFFF;
   }
 
-  /** True if the tag has a slot, below {@link #slotCount()}. */
+  /**
+   * True if the tag has a slot, below {@link #slotCount()}. False for {@code 0L}, the id {@link
+   * #keyOf} gives a custom tag, which would otherwise decode as slot 0.
+   */
   public static boolean hasSlot(long tagId) {
-    return slotOf(tagId) != NO_SLOT;
+    return tagId != 0L && slotOf(tagId) != NO_SLOT;
   }
 
   /** Slots in use: every slotted tag's slot is below this. */
 
@@ -67,11 +67,18 @@ void theSlotDoesNotChangeTheSerialOrFlags() {
     assertFalse(KnownTagCodec.isTraceLevel(tagId));
   }
 
+  @Test
+  void aCustomTagHasNoSlot() {
+    // keyOf gives a custom tag the id 0L, whose slot bits decode as slot 0
+    assertEquals(0L, KnownTagCodec.keyOf("my.custom.tag"));
+    assertFalse(KnownTagCodec.hasSlot(0L));
+  }
+
   @Test
   void anUnslottedTagSharesTheReservedTopBit() {
     // An occupancy bit of 1L << slotOf(id) needs no branch for NO_SLOT: a long shift uses only the
     // low 6 bits of its count, so 0xFFFF lands on bit 63, which no real slot uses
     assertEquals(1L << 63, 1L << KnownTagCodec.slotOf(KnownTags.RESOURCE_NAME_ID));
-    assertTrue(KnownTagCodec.slotCount() < 63, "real slots must stay below the shared bit 63");
+    assertTrue(KnownTagCodec.slotCount() <= 63, "real slots must stay below the shared bit 63");
   }
 }
Original file line number	Diff line number	Diff line change
`@@ -81,9 +81,12 @@ public static int slotOf(long tagId) {`
`81`	`81`	`return (int) (tagId >>> SLOT_SHIFT) & 0xFFFF;`
`82`	`82`	`}`
`83`	`83`
`84`		`- /** True if the tag has a slot, below {@link #slotCount()}. */`
	`84`	`+ /**`
	`85`	`+ * True if the tag has a slot, below {@link #slotCount()}. False for {@code 0L}, the id {@link`
	`86`	`+ * #keyOf} gives a custom tag, which would otherwise decode as slot 0.`
	`87`	`+ */`
`85`	`88`	`public static boolean hasSlot(long tagId) {`
`86`		`- return slotOf(tagId) != NO_SLOT;`
	`89`	`+ return tagId != 0L && slotOf(tagId) != NO_SLOT;`
`87`	`90`	`}`
`88`	`91`
`89`	`92`	`/** Slots in use: every slotted tag's slot is below this. */`
Original file line number	Diff line number	Diff line change
`@@ -67,11 +67,18 @@ void theSlotDoesNotChangeTheSerialOrFlags() {`
`67`	`67`	`assertFalse(KnownTagCodec.isTraceLevel(tagId));`
`68`	`68`	`}`
`69`	`69`
	`70`	`+ @Test`
	`71`	`+ void aCustomTagHasNoSlot() {`
	`72`	`+ // keyOf gives a custom tag the id 0L, whose slot bits decode as slot 0`
	`73`	`+ assertEquals(0L, KnownTagCodec.keyOf("my.custom.tag"));`
	`74`	`+ assertFalse(KnownTagCodec.hasSlot(0L));`
	`75`	`+ }`
	`76`	`+`
`70`	`77`	`@Test`
`71`	`78`	`void anUnslottedTagSharesTheReservedTopBit() {`
`72`	`79`	`// An occupancy bit of 1L << slotOf(id) needs no branch for NO_SLOT: a long shift uses only the`
`73`	`80`	`// low 6 bits of its count, so 0xFFFF lands on bit 63, which no real slot uses`
`74`	`81`	`assertEquals(1L << 63, 1L << KnownTagCodec.slotOf(KnownTags.RESOURCE_NAME_ID));`
`75`		`- assertTrue(KnownTagCodec.slotCount() < 63, "real slots must stay below the shared bit 63");`
	`82`	`+ assertTrue(KnownTagCodec.slotCount() <= 63, "real slots must stay below the shared bit 63");`
`76`	`83`	`}`
`77`	`84`	`}`