bug #21647 [Yaml] consistently parse omitted keys as the colon (xabbuh)

This PR was merged into the 3.2 branch.

Discussion
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[Yaml] consistently parse omitted keys as the colon

| Q             | A
| ------------- | ---
| Branch?       | 3.2
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

Before the changes made in #20335, an empty mapping key was parsed as `:`. This behaviour is not correct according to the spec, but we should keep the parser behaviour consistent to not break backward compatibility (I will deprecate it in a different PR on `master`).

Commits
-------

e2ebecc consistently parse omitted keys as the colon

src/Symfony/Component/Yaml/Inline.php

@@ -459,11 +459,11 @@ private static function parseMapping($mapping, $flags, &$i = 0, $references = ar
             // key
             $key = self::parseScalar($mapping, $flags, array(':', ' '), array('"', "'"), $i, false);
 
-            if (false === $i = strpos($mapping, ':', $i)) {
+            if (':' !== $key && false === $i = strpos($mapping, ':', $i)) {
                 break;
             }
 
-            if (!isset($mapping[$i + 1]) || !in_array($mapping[$i + 1], array(' ', ',', '[', ']', '{', '}'), true)) {
+            if (':' !== $key && (!isset($mapping[$i + 1]) || !in_array($mapping[$i + 1], array(' ', ',', '[', ']', '{', '}'), true))) {
                 @trigger_error('Using a colon that is not followed by an indication character (i.e. " ", ",", "[", "]", "{", "}" is deprecated since version 3.2 and will throw a ParseException in 4.0.', E_USER_DEPRECATED);
             }
 

src/Symfony/Component/Yaml/Tests/InlineTest.php

@@ -686,4 +686,9 @@ public function testVeryLongQuotedStrings()
 
         $this->assertEquals($longStringWithQuotes, $arrayFromYaml['longStringWithQuotes']);
     }
+
+    public function testOmittedMappingKeyIsParsedAsColon()
+    {
+        $this->assertSame(array(':' => 'foo'), Inline::parse('{: foo}'));
+    }
 }