Deprecate duplicate mapping keys containing null

Author: xabbuh

CHANGELOG.md

@@ -4,7 +4,7 @@ CHANGELOG
 7.2
 ---
 
- * Duplicate mapping keys throw a `ParseException` even when such key is `null`
+ * Deprecate parsing duplicate mapping keys whose value is `null`
 
 7.1
 ---

Parser.php

@@ -299,7 +299,11 @@ private function doParse(string $value, int $flags): mixed
                     if (!$this->isNextLineIndented() && !$this->isNextLineUnIndentedCollection()) {
                         // Spec: Keys MUST be unique; first one wins.
                         // But overwriting is allowed when a merge node is used in current block.
-                        if ($allowOverwrite || !\array_key_exists($key, $data)) {
+                        if ($allowOverwrite || !isset($data[$key])) {
+                            if (!$allowOverwrite && array_key_exists($key, $data)) {
+                                trigger_deprecation('symfony/yaml', '7.2', 'Duplicate key "%s" detected on line %d whilst parsing YAML. Silent handling of duplicate mapping keys in YAML is deprecated and will throw a ParseException in 8.0.', $key, $this->getRealCurrentLineNb() + 1);
+                            }
+
                             if (null !== $subTag) {
                                 $data[$key] = new TaggedValue($subTag, '');
                             } else {
@@ -320,7 +324,11 @@ private function doParse(string $value, int $flags): mixed
                             }
 
                             $data += $value;
-                        } elseif ($allowOverwrite || !\array_key_exists($key, $data)) {
+                        } elseif ($allowOverwrite || !isset($data[$key])) {
+                            if (!$allowOverwrite && array_key_exists($key, $data)) {
+                                trigger_deprecation('symfony/yaml', '7.2', 'Duplicate key "%s" detected on line %d whilst parsing YAML. Silent handling of duplicate mapping keys in YAML is deprecated and will throw a ParseException in 8.0.', $key, $this->getRealCurrentLineNb() + 1);
+                            }
+
                             // Spec: Keys MUST be unique; first one wins.
                             // But overwriting is allowed when a merge node is used in current block.
                             if (null !== $subTag) {
@@ -336,7 +344,11 @@ private function doParse(string $value, int $flags): mixed
                     $value = $this->parseValue(rtrim($values['value']), $flags, $context);
                     // Spec: Keys MUST be unique; first one wins.
                     // But overwriting is allowed when a merge node is used in current block.
-                    if ($allowOverwrite || !\array_key_exists($key, $data)) {
+                    if ($allowOverwrite || !isset($data[$key])) {
+                        if (!$allowOverwrite && array_key_exists($key, $data)) {
+                            trigger_deprecation('symfony/yaml', '7.2', 'Duplicate key "%s" detected on line %d whilst parsing YAML. Silent handling of duplicate mapping keys in YAML is deprecated and will throw a ParseException in 8.0.', $key, $this->getRealCurrentLineNb() + 1);
+                        }
+
                         $data[$key] = $value;
                     } else {
                         throw new ParseException(\sprintf('Duplicate key "%s" detected.', $key), $this->getRealCurrentLineNb() + 1, $this->currentLine);

Tests/ParserTest.php

@@ -12,13 +12,16 @@
 namespace Symfony\Component\Yaml\Tests;
 
 use PHPUnit\Framework\TestCase;
+use Symfony\Bridge\PhpUnit\ExpectDeprecationTrait;
 use Symfony\Component\Yaml\Exception\ParseException;
 use Symfony\Component\Yaml\Parser;
 use Symfony\Component\Yaml\Tag\TaggedValue;
 use Symfony\Component\Yaml\Yaml;
 
 class ParserTest extends TestCase
 {
+    use ExpectDeprecationTrait;
+
     private ?Parser $parser;
 
     protected function setUp(): void
@@ -1026,15 +1029,24 @@ public static function getParseExceptionOnDuplicateData()
 EOD;
         $tests[] = [$yaml, 'child_sequence', 6];
 
+        return $tests;
+    }
+
+    /**
+     * @group legacy
+     */
+    public function testNullAsDuplicatedData()
+    {
+        $this->expectDeprecation('Since symfony/yaml 7.2: Duplicate key "child" detected on line 4 whilst parsing YAML. Silent handling of duplicate mapping keys in YAML is deprecated and will throw a ParseException in 8.0.');
+
         $yaml = <<<EOD
 parent:
   child:
   child2:
   child:
 EOD;
-        $tests[] = [$yaml, 'child', 4];
 
-        return $tests;
+        Yaml::parse($yaml);
     }
 
     public function testEmptyValue()

composer.json

@@ -17,6 +17,7 @@
     ],
     "require": {
         "php": ">=8.2",
+        "symfony/deprecation-contracts": "^2.5|^3.0",
         "symfony/polyfill-ctype": "^1.8"
     },
     "require-dev": {