Added support for null coalescing operator

Fixes #882

CHANGELOG.md

@@ -10,7 +10,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Added support for PHP8.2
 - Added a new way to extend Smarty functionality using `Smarty::addExtension()`. Please see the docs for more information.
 - Custom tags can accept positional parameters, so you can write a block compiler that support this: `{trans "Jack" "dull boy"}All work and no play makes %s a %s.{/trans}` [#164](https://github.com/smarty-php/smarty/issues/164)
-- Full support for ternary operator: `{$test ? $a : $b}` and `{$var ?: $alternative}` [#881](https://github.com/smarty-php/smarty/issues/881)
+- Full support for ternary operator: `{$test ? $a : $b}` and `{$var ?: $value_if_falsy}` [#881](https://github.com/smarty-php/smarty/issues/881)
+- Full support for null coalescing operator: `{$var ?? $value_if_null}` [#882](https://github.com/smarty-php/smarty/issues/882)
 
 ### Changed
 - All Smarty code is now in the \Smarty namespace. For simple use-cases, you only need to add

src/Parser/TemplateParser.y

@@ -617,6 +617,11 @@ expr(res)        ::= value(v). {
     res = v;
 }
 
+                 // nullcoalescing
+expr(res)        ::= nullcoalescing(v). {
+    res = v;
+}
+
                  // ternary
 expr(res)        ::= ternary(v). {
     res = v;
@@ -668,6 +673,10 @@ expr(res)        ::= expr(e1) ISIN value(v).  {
     res = 'in_array('.e1.',(array)'.v.')';
 }
 
+// null coalescing
+nullcoalescing(res)        ::= expr(v) QMARK QMARK expr(e2). {
+    res = v.' ?? '.e2;
+}
 
 //
 // ternary

tests/UnitTests/TemplateSource/NullCoalescingTest.php

@@ -0,0 +1,34 @@
+<?php
+
+class NullCoalescingTest extends PHPUnit_Smarty {
+
+	public function setUp(): void
+	{
+		$this->setUpSmarty('/tmp');
+		$this->cleanDirs();
+	}
+
+	public function testUndefined() {
+		$tpl = $this->smarty->createTemplate('string:{$myvar ?? "undefined"}');
+		$this->assertEquals('undefined', $this->smarty->fetch($tpl));
+	}
+
+	/**
+	 * @dataProvider dataForOther
+	 */
+	public function testOther($value, $expected) {
+		$tpl = $this->smarty->createTemplate('string:{$myvar ?? "undefined"}');
+		$tpl->assign('myvar', $value);
+		$this->assertEquals($expected, $this->smarty->fetch($tpl));
+	}
+
+	public function dataForOther() {
+		return [
+			[null, 'undefined'],
+			['blah', ''],
+			['', ''],
+			[false, false],
+		];
+	}
+
+}