properly catch legacy tag syntax usages

Author: xabbuh

Inline.php

@@ -670,18 +670,22 @@ private static function parseTag(string $value, int &$i, int $flags): ?string
         $nextOffset += strspn($value, ' ', $nextOffset);
 
         // Is followed by a scalar and is a built-in tag
-        if ($tag && (!isset($value[$nextOffset]) || !\in_array($value[$nextOffset], ['[', '{'], true)) && ('!' === $tag[0] || 'str' === $tag || 'php/const' === $tag || 'php/object' === $tag)) {
+        if ('' !== $tag && (!isset($value[$nextOffset]) || !\in_array($value[$nextOffset], ['[', '{'], true)) && ('!' === $tag[0] || 'str' === $tag || 'php/const' === $tag || 'php/object' === $tag)) {
             // Manage in {@link self::evaluateScalar()}
             return null;
         }
 
         $i = $nextOffset;
 
         // Built-in tags
-        if ($tag && '!' === $tag[0]) {
+        if ('' !== $tag && '!' === $tag[0]) {
             throw new ParseException(sprintf('The built-in tag "!%s" is not implemented.', $tag), self::$parsedLineNumber + 1, $value, self::$parsedFilename);
         }
 
+        if ('' !== $tag && !isset($value[$i])) {
+            throw new ParseException(sprintf('Missing value for tag "%s".', $tag), self::$parsedLineNumber + 1, $value, self::$parsedFilename);
+        }
+
         if ('' === $tag || Yaml::PARSE_CUSTOM_TAGS & $flags) {
             return $tag;
         }

Tests/ParserTest.php

@@ -12,6 +12,7 @@
 namespace Symfony\Component\Yaml\Tests;
 
 use PHPUnit\Framework\TestCase;
+use Symfony\Component\Yaml\Exception\ParseException;
 use Symfony\Component\Yaml\Parser;
 use Symfony\Component\Yaml\Tag\TaggedValue;
 use Symfony\Component\Yaml\Yaml;
@@ -1841,6 +1842,14 @@ public function testPhpConstantTagMappingKey()
         $this->assertSame($expected, $this->parser->parse($yaml, Yaml::PARSE_CONSTANT));
     }
 
+    public function testDeprecatedPhpConstantSyntax()
+    {
+        $this->expectException(ParseException::class);
+        $this->expectExceptionMessage('Missing value for tag "php/const:App\Kernel::SEMART_VERSION" at line 1 (near "!php/const:App\Kernel::SEMART_VERSION").');
+
+        $this->parser->parse('!php/const:App\Kernel::SEMART_VERSION', Yaml::PARSE_CUSTOM_TAGS | Yaml::PARSE_CONSTANT);
+    }
+
     public function testMergeKeysWhenMappingsAreParsedAsObjects()
     {
         $yaml = <<<YAML