https://git.ruby-lang.org/ruby.git/commit/?id=fc83b4896e

From fc83b4896e9d7de084b203b133b84c1209c6ad88 Mon Sep 17 00:00:00 2001
From: Jeremy Evans <code@j...>
Date: Tue, 9 Feb 2021 14:15:31 -0800
Subject: Don't document that Thread#group can return nil

Thread's are assigned a group at initialization, and no API exists
for them to unassign them from a group without assigning them to
another group.

Fixes [Bug #17505]
---
 thread.c | 10 ++--------
 1 file changed, 2 insertions(+), 8 deletions(-)

diff --git a/thread.c b/thread.c
index 1e35ad4..c600a87 100644
--- a/thread.c
+++ b/thread.c
@@ -3213,8 +3213,7 @@ rb_thread_report_exc_set(VALUE thread, VALUE val) https://github.com/ruby/ruby/blob/trunk/thread.c#L3213
  *  call-seq:
  *     thr.group   -> thgrp or nil
  *
- *  Returns the ThreadGroup which contains the given thread, or returns +nil+
- *  if +thr+ is not a member of any group.
+ *  Returns the ThreadGroup which contains the given thread.
  *
  *     Thread.main.group   #=> #<ThreadGroup:0x4029d914>
  */
@@ -3222,8 +3221,7 @@ rb_thread_report_exc_set(VALUE thread, VALUE val) https://github.com/ruby/ruby/blob/trunk/thread.c#L3221
 VALUE
 rb_thread_group(VALUE thread)
 {
-    VALUE group = rb_thread_ptr(thread)->thgroup;
-    return group == 0 ? Qnil : group;
+    return rb_thread_ptr(thread)->thgroup;
 }
 
 static const char *
@@ -4974,10 +4972,6 @@ thgroup_add(VALUE group, VALUE thread) https://github.com/ruby/ruby/blob/trunk/thread.c#L4972
 	rb_raise(rb_eThreadError, "can't move to the enclosed thread group");
     }
 
-    if (!target_th->thgroup) {
-	return Qnil;
-    }
-
     if (OBJ_FROZEN(target_th->thgroup)) {
 	rb_raise(rb_eThreadError, "can't move from the frozen thread group");
     }
-- 
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