bug #37744 [Yaml] Fix for #36624; Allow PHP constant as first key in block (jnye)

This PR was squashed before being merged into the 3.4 branch.

Discussion
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[Yaml] Fix for #36624; Allow PHP constant as first key in block

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | Fix #36624
| License       | MIT
| Doc PR        |

When using a PHP constant in the first key of a mapping the parser would throw an exception. However if you used a PHP constant in any other key but the first it was allowed. This fix allows PHP constant as the first key.

I've included a test for this parser error along with the fix.

Commits
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46f635c492 [Yaml] Fix for #36624; Allow PHP constant as first key in block

Author: fabpot

Parser.php

@@ -230,8 +230,12 @@ private function doParse($value, $flags)
                         $this->parseBlock($this->getRealCurrentLineNb() + 1, $this->getNextEmbedBlock(null, true), $flags)
                     );
                 } else {
-                    if (isset($values['leadspaces'])
-                        && self::preg_match('#^(?P<key>'.Inline::REGEX_QUOTED_STRING.'|[^ \'"\{\[].*?) *\:(\s+(?P<value>.+?))?\s*$#u', $this->trimTag($values['value']), $matches)
+                    if (
+                        isset($values['leadspaces'])
+                        && (
+                            '!' === $values['value'][0]
+                            || self::preg_match('#^(?P<key>'.Inline::REGEX_QUOTED_STRING.'|[^ \'"\{\[].*?) *\:(\s+(?P<value>.+?))?\s*$#u', $this->trimTag($values['value']), $matches)
+                        )
                     ) {
                         // this is a compact notation element, add to next block and parse
                         $block = $values['value'];

Tests/ParserTest.php

@@ -2025,6 +2025,34 @@ public function testPhpConstantTagMappingKeyWithKeysCastToStrings()
         $this->assertSame($expected, $this->parser->parse($yaml, Yaml::PARSE_CONSTANT | Yaml::PARSE_KEYS_AS_STRINGS));
     }
 
+    public function testPhpConstantTagMappingAsScalarKey()
+    {
+        $yaml = <<<YAML
+map1:
+  - foo: 'value_0'
+    !php/const 'Symfony\Component\Yaml\Tests\B::BAR': 'value_1'
+map2:
+  - !php/const 'Symfony\Component\Yaml\Tests\B::FOO': 'value_0'
+    bar: 'value_1'
+YAML;
+        $this->assertSame([
+            'map1' => [['foo' => 'value_0', 'bar' => 'value_1']],
+            'map2' => [['foo' => 'value_0', 'bar' => 'value_1']],
+        ], $this->parser->parse($yaml, Yaml::PARSE_CONSTANT));
+    }
+
+    public function testTagMappingAsScalarKey()
+    {
+        $yaml = <<<YAML
+map1:
+  - !!str 0: 'value_0'
+    !!str 1: 'value_1'
+YAML;
+        $this->assertSame([
+            'map1' => [['0' => 'value_0', '1' => 'value_1']],
+        ], $this->parser->parse($yaml));
+    }
+
     public function testMergeKeysWhenMappingsAreParsedAsObjects()
     {
         $yaml = <<<YAML
@@ -2287,7 +2315,7 @@ public function testMultiLineComment()
 parameters:
     abc
 
-# Comment 
+# Comment
 YAML;
 
         $this->assertSame(['parameters' => 'abc'], $this->parser->parse($yaml));