Dumper.php

@@ -80,7 +80,7 @@ public function dump($input, int $inline = 0, int $indent = 0, int $flags = 0):
                 if ($value instanceof TaggedValue) {
                     $output .= sprintf('%s%s !%s', $prefix, $dumpAsMap ? Inline::dump($key, $flags).':' : '-', $value->getTag());
 
-                    if ($inline - 1 <= 0) {
+                    if ($inline - 1 <= 0 || null === $value->getValue() || is_scalar($value->getValue())) {
                         $output .= ' '.$this->dump($value->getValue(), $inline - 1, 0, $flags)."\n";
                     } else {
                         $output .= "\n";

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/DumperTest.php

@@ -459,6 +459,34 @@ public function testDumpingTaggedValueMapWithInlinedTagValues()
         $this->assertSame($expected, $yaml);
     }
 
+    public function testDumpingNotInlinedScalarTaggedValue()
+    {
+        $data = [
+            'user1' => new TaggedValue('user', 'jane'),
+            'user2' => new TaggedValue('user', 'john'),
+        ];
+        $expected = <<<YAML
+user1: !user jane
+user2: !user john
+
+YAML;
+
+        $this->assertSame($expected, $this->dumper->dump($data, 2));
+    }
+
+    public function testDumpingNotInlinedNullTaggedValue()
+    {
+        $data = [
+            'foo' => new TaggedValue('bar', null),
+        ];
+        $expected = <<<YAML
+foo: !bar null
+
+YAML;
+
+        $this->assertSame($expected, $this->dumper->dump($data, 2));
+    }
+
     public function testDumpMultiLineStringAsScalarBlock()
     {
         $data = [

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