Unary null coalescing operator

spec/10-expressions.md

@@ -2964,11 +2964,13 @@ function factorial($int)
 <!-- GRAMMAR
 coalesce-expression:
   logical-inc-OR-expression-1 '??' expression
+  logical-inc-OR-expression-1 '??'
 -->
 
 <pre>
 <i id="grammar-coalesce-expression">coalesce-expression:</i>
    <i><a href="#grammar-logical-inc-OR-expression-1">logical-inc-OR-expression-1</a></i>   ??   <i><a href="#grammar-expression">expression</a></i>
+   <i><a href="#grammar-logical-inc-OR-expression-1">logical-inc-OR-expression-1</a></i>   ??
 </pre>
 
 **Semantics**
@@ -2978,6 +2980,8 @@ Given the expression `e1 ?? e2`, if `e1` is set and not `NULL` (i.e. TRUE for
 evaluated, and the result becomes the result of the whole
 expression. There is a sequence point after the evaluation of `e1`.
 
+Given the expression `e1??`, its semantics are equivalent to `e1 ?? e2` where `e2` is the constant value `NULL`.
+
 Note that the semantics of `??` is similar to `isset` so that uninitialized variables will not produce
 warnings when used in `e1`.
 
@@ -2990,12 +2994,18 @@ $arr = ["foo" => "bar", "qux" => NULL];
 $obj = (object)$arr;
 
 $a = $arr["foo"] ?? "bang"; // "bar" as $arr["foo"] is set and not NULL
+$a = $arr["foo"]??;         // "bar" as $arr["foo"] is set and not NULL
 $a = $arr["qux"] ?? "bang"; // "bang" as $arr["qux"] is NULL
+$a = $arr["qux"]??;         // "bang" as $arr["qux"] is NULL
 $a = $arr["bing"] ?? "bang"; // "bang" as $arr["bing"] is not set
+$a = $arr["bing"]??;         // NULL as $arr["bing"] is not set
 
 $a = $obj->foo ?? "bang"; // "bar" as $obj->foo is set and not NULL
+$a = $obj->foo??;         // "bar" as $obj->foo is set and not NULL
 $a = $obj->qux ?? "bang"; // "bang" as $obj->qux is NULL
+$a = $obj->qux??;         // NULL as $obj->qux is NULL
 $a = $obj->bing ?? "bang"; // "bang" as $obj->bing is not set
+$a = $obj->bing??;         // NULL as $obj->bing is not set
 
 $a = NULL ?? $arr["bing"] ?? 2; // 2 as NULL is NULL, and $arr["bing"] is not set
 

spec/19-grammar.md

@@ -660,6 +660,7 @@ The grammar notation is described in [Grammars section](09-lexical-structure.md#
 
 <i id="grammar-coalesce-expression">coalesce-expression:</i>
    <i><a href="#grammar-logical-inc-OR-expression-1">logical-inc-OR-expression-1</a></i>   ??   <i><a href="#grammar-expression">expression</a></i>
+   <i><a href="#grammar-logical-inc-OR-expression-1">logical-inc-OR-expression-1</a></i>   ??
 
 <i id="grammar-assignment-expression">assignment-expression:</i>
    <i><a href="#grammar-conditional-expression">conditional-expression</a></i>

tests/expressions/coalesce_operator/unary.phpt

@@ -0,0 +1,78 @@
+--TEST--
+Test ?? operator (unary)
+--FILE--
+<?php
+
+$var = 7;
+$var2 = NULL;
+
+$obj = new StdClass;
+$obj->boo = 7;
+
+$arr = [
+	2 => 7,
+	"foo" => "bar",
+	"foobar" => NULL,
+	"qux" => $obj,
+	"bing" => [
+		"bang"
+	]
+];
+
+function foobar() {
+	echo "called\n";
+	return ['a'];
+}
+
+var_dump($nonexistent_variable??);
+echo PHP_EOL;
+var_dump($var??);
+var_dump($var2??);
+echo PHP_EOL;
+var_dump($obj->boo??);
+var_dump($obj->bing??);
+var_dump($arr["qux"]->boo??);
+var_dump($arr["qux"]->bing??);
+echo PHP_EOL;
+var_dump($arr[2]??);
+var_dump($arr["foo"]??);
+var_dump($arr["foobar"]??);
+var_dump($arr["qux"]??);
+var_dump($arr["bing"][0]??);
+var_dump($arr["bing"][1]??);
+echo PHP_EOL;
+var_dump(foobar()[0]??);
+echo PHP_EOL;
+function f($x)
+{
+    printf("%s(%d)\n", __FUNCTION__, $x);
+    return $x;
+}
+
+$a = f(null)??;
+?>
+--EXPECTF--
+NULL
+
+int(7)
+NULL
+
+int(7)
+NULL
+int(7)
+NULL
+
+int(7)
+string(3) "bar"
+NULL
+object(stdClass)#%d (%d) {
+  ["boo"]=>
+  int(7)
+}
+string(4) "bang"
+NULL
+
+called
+string(1) "a"
+
+f(0)