Merge branch '3.4' into 4.4

* 3.4:
  [Yaml] Fix for #36624; Allow PHP constant as first key in block
  Use PHPUnit 9.3 on php 8.
  fix mapping errors from unmapped forms
  [Validator] Add target guards for Composite nested constraints

Author: fabpot

Parser.php

@@ -176,8 +176,12 @@ private function doParse(string $value, int $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

@@ -2077,6 +2077,34 @@ public function testDeprecatedPhpConstantSyntax()
         $this->parser->parse('!php/const:App\Kernel::SEMART_VERSION', Yaml::PARSE_CUSTOM_TAGS | Yaml::PARSE_CONSTANT);
     }
 
+    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
@@ -2339,7 +2367,7 @@ public function testMultiLineComment()
 parameters:
     abc
 
-# Comment 
+# Comment
 YAML;
 
         $this->assertSame(['parameters' => 'abc'], $this->parser->parse($yaml));