From 397d810c59d1e7cfefb29944fa6e0eab15cc19ab Mon Sep 17 00:00:00 2001
From: Michael Kay <mike@saxonica.com>
Date: Sat, 4 May 2024 23:13:27 +0100
Subject: [PATCH] When coercing to a union type, atomize first before
 considering alternatives.

---
 specifications/xquery-40/src/expressions.xml | 28 +++++++++++++++++---
 1 file changed, 25 insertions(+), 3 deletions(-)

diff --git a/specifications/xquery-40/src/expressions.xml b/specifications/xquery-40/src/expressions.xml
index 025ce0d7a..2d9b26704 100644
--- a/specifications/xquery-40/src/expressions.xml
+++ b/specifications/xquery-40/src/expressions.xml
@@ -7024,7 +7024,29 @@ name.</p>
                sequence, given a required <termref def="dt-item-type"/> <var>R</var>.</p>
                
                <olist>
-                  <item><p>If <var>R</var> is a <termref def="dt-choice-item-type"/> or
+                  <item><p>If <var>R</var> is a <termref def="dt-generalized-atomic-type"/>
+                     (for example, if it is an <termref def="dt-atomic-type"/>,
+                     a <termref def="dt-pure-union-type"/>, or
+                     an <termref def="dt-enumeration-type"/>), and <var>J</var>
+                     is not an atomic value, then:
+                     <olist>
+                        <item><p><var>J</var> is atomized to produce a sequence of atomic values
+                        <var>JJ</var>.</p></item>
+                        <item><p>Each atomic value in <var>JJ</var> is coerced to the required
+                        type <var>R</var> by recursive application of the item coercion rules (the 
+                        rules in this section) to produce a value <var>V</var>.</p></item>
+                        <item><p>The result is the sequence-concatenation of the <var>V</var> values.</p></item>
+                     </olist>
+                  </p>
+                  <note><p>For example, if <code>J</code> is an element with type annotation
+                  <code>xs:integer</code>, and <code>R</code> is the union type <code>xs:numeric</code>,
+                  then the effect is to atomize the element to an <code>xs:integer</code>, and then
+                  to coerce the resulting <code>xs:integer</code> to <code>xs:numeric</code>
+                  (which leaves the integer unchanged). This is not the same as attempting to
+                  coerce the element to each of the alternatives of the union type in turn, which
+                  would deliver an instance of <code>xs:double</code>.</p></note>
+                  </item>
+                  <item><p>Otherwise, if <var>R</var> is a <termref def="dt-choice-item-type"/> or
                      a <termref def="dt-pure-union-type"/> (which includes the case where
                      it is an <termref def="dt-enumeration-type"/>), then:</p>
                      
@@ -7086,7 +7108,7 @@ name.</p>
                         <termref def="dt-choice-item-type">choice item types.</termref></p></note>
                   </item>
                   
-                  <item>
+                  <!--<item>
                      <p>If <var>R</var> is an <termref def="dt-atomic-type"/>
                         and <var>J</var> is not an atomic value,
                         then <var>J</var> is <termref def="dt-atomization">atomized</termref>. 
@@ -7096,7 +7118,7 @@ name.</p>
                         these atomic values in turn, with the same item type <var>R</var>
                         as the required type, and the results are concatenated
                         in order to form the result sequence.</p>
-                  </item>
+                  </item>-->
                         
                   <item>
                      <p>If <var>R</var> is an <termref def="dt-atomic-type"/>