[BUGFIX] Fix the return type of Selector::isValid() (#1174)

Fixes #1043
Part of #811

Author: oliverklee

CHANGELOG.md

@@ -34,7 +34,7 @@ Please also have a look at our
 - Use more native type declarations and strict mode
   (#641, #772, #774, #778, #804, #841, #873, #875, #891, #922, #923, #933, #958,
   #964, #967, #1000, #1044, #1134, #1136, #1137, #1139, #1140, #1141, #1145,
-  #1162, #1163, #1166, #1172)
+  #1162, #1163, #1166, #1172, #1174)
 - Add visibility to all class/interface constants (#469)
 
 ### Deprecated

src/Property/Selector.php

@@ -37,13 +37,14 @@ class Selector implements Renderable
     private $selector;
 
     /**
-     * @return bool
-     *
      * @internal since V8.8.0
      */
-    public static function isValid(string $selector)
+    public static function isValid(string $selector): bool
     {
-        return \preg_match(static::SELECTOR_VALIDATION_RX, $selector);
+        // Note: We need to use `static::` here as the constant is overridden in the `KeyframeSelector` class.
+        $numberOfMatches = \preg_match(static::SELECTOR_VALIDATION_RX, $selector);
+
+        return $numberOfMatches === 1;
     }
 
     public function __construct(string $selector)

tests/Unit/Property/SelectorTest.php

@@ -98,4 +98,44 @@ public function getSpecificityReturnsSpecificityOfSelectorLastProvidedViaSetSele
 
         self::assertSame($expectedSpecificity, $subject->getSpecificity());
     }
+
+    /**
+     * @test
+     *
+     * @dataProvider provideSelectorsAndSpecificities
+     */
+    public function isValidForValidSelectorReturnsTrue(string $selector): void
+    {
+        self::assertTrue(Selector::isValid($selector));
+    }
+
+    /**
+     * @return array<string, array{0: string}>
+     */
+    public static function provideInvalidSelectors(): array
+    {
+        return [
+            // This is currently broken.
+            // 'empty string' => [''],
+            'percent sign' => ['%'],
+            // This is currently broken.
+            // 'hash only' => ['#'],
+            // This is currently broken.
+            // 'dot only' => ['.'],
+            'slash' => ['/'],
+            'less-than sign' => ['<'],
+            // This is currently broken.
+            // 'whitespace only' => [" \t\n\r"],
+        ];
+    }
+
+    /**
+     * @test
+     *
+     * @dataProvider provideInvalidSelectors
+     */
+    public function isValidForInvalidSelectorReturnsFalse(string $selector): void
+    {
+        self::assertFalse(Selector::isValid($selector));
+    }
 }