Computed attribute value in the xml literal requires explicit call to the toString() method.

[scabug]

It looks inconsistent when values inside the xml elements are implicitly converted to string but values in the xml attributes are not.

Reproducing:

scala> <x>{2+2}</x>
 
res2: scala.xml.Elem = <x>4</x>

scala> <x y={2+2} />

<console>:5: error: overloaded method constructor UnprefixedAttribute
with alternatives 
(String,Option[Seq[scala.xml.Node]],scala.xml.MetaData)scala.xml.UnprefixedAttribute 
<and> 
(String,String,scala.xml.MetaData)scala.xml.UnprefixedAttribute 
<and> (String,Seq[scala.xml.Node],scala.xml.MetaData)scala.xml.UnprefixedAttribute cannot be applied to (java.lang.String,Int,scala.xml.MetaData)
       <x y={2+2} />
        ^

scala> <x y={(2+2).toString} /> 
 
res4: scala.xml.Elem = <x y="4"></x>

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-1787?orig=1
Reporter: Dan (lifecoder)

[scabug]

@retronym said (edited on May 20, 2012 8:12:06 AM UTC):
Notes in passing:

https://github.com/scala/scala/blob/8c95273b/src/compiler/scala/tools/nsc/ast/parser/SymbolicXMLBuilder.scala#L216

Elaborating on that comment, simply adding toString here would not preserve these:

scala> <a href={None}/>
res6: scala.xml.Elem = <a/>

scala> <a href={Some(new xml.Atom("xxx"))}/>
res7: scala.xml.Elem = <a href="xxx"/>

The desugaring:

$ scala -Xprint:parser -e "<a b={1+1}>{1+1}</a>"

              var $md: _root_.scala.xml.MetaData = _root_.scala.xml.Null;
              $md = new _root_.scala.xml.UnprefixedAttribute("b", 1.$plus(1), $md);
              new _root_.scala.xml.Elem(null, "a", $md, $scope, ({
                val $buf = new _root_.scala.xml.NodeBuffer();
                $buf.$amp$plus(1.$plus(1));
                $buf
              }: _*))

For elements, the implicit toString comes (eventually) because the added expressions are routed through NodeBuffer#&+.(https://github.com/scala/scala/blob/8c95273b/src/library/scala/xml/NodeBuffer.scala)

But it's really hard to retrofit this for attributes without (theoretically) breaking existing code (in particular becase the constructor for UnprefixedAttribute is overloaded).

[scabug]

@SethTisue said:
The scala-xml library is now community-maintained. Issues with it are now tracked at https://github.com/scala/scala-xml/issues instead of here in the Scala JIRA.

Interested community members: if you consider this issue significant, feel free to open a new issue for it on GitHub, with links in both directions.