Tokenizer/PHP: arrow function tokenization broken when true/false used in return type

Since PHP 8.0, `false` and `null` can be included in a union return type.
As of PHP 8.2, both `true`, `false` and `null` can be used as a stand-alone return type.

The tokenizer layer handling arrow functions did not take this into account correctly. While `null` was handled correctly, `true` and `false` was not and would result in the arrow function `fn` keyword being tokenized as `T_STRING` across all PHP versions.
As a result of that, the other typical tokenizer changes related to arrow functions (`=>` as `T_FN_ARROW`, scope/parenthesis owners etc) would also not be executed correctly.

In practice, I suspect few people will have run into this bug as, after all, what's the point of declaring an arrow function which will only ever return `true` or `false` ? so in practice, it is likely to only have come into play for people using `true` or `false` as part of an arrow function union type.
All the same, PHPCS should handle this correctly.

Includes unit tests proving the bug and safeguarding the fix.

Author: jrfnl

src/Tokenizers/PHP.php

@@ -2540,6 +2540,8 @@ protected function processAdditional()
                         T_NAME_QUALIFIED       => T_NAME_QUALIFIED,
                         T_NAME_RELATIVE        => T_NAME_RELATIVE,
                         T_NULL                 => T_NULL,
+                        T_TRUE                 => T_TRUE,
+                        T_FALSE                => T_FALSE,
                         T_NULLABLE             => T_NULLABLE,
                         T_PARENT               => T_PARENT,
                         T_SELF                 => T_SELF,

tests/Core/Tokenizer/BackfillFnTokenTest.inc

@@ -98,12 +98,27 @@ fn(array $a) : array => $a;
 /* testStaticReturnType */
 fn(array $a) : static => $a;
 
+/* testFalseReturnType */
+fn(array $a) : false => $a;
+
+/* testTrueReturnType */
+fn(array $a) : True => $a;
+
+/* testNullReturnType */
+fn(array $a) : null => $a;
+
 /* testUnionParamType */
 $arrowWithUnionParam = fn(int|float $param) : SomeClass => new SomeClass($param);
 
 /* testUnionReturnType */
 $arrowWithUnionReturn = fn($param) : int|float => $param | 10;
 
+/* testUnionReturnTypeWithTrue */
+$arrowWithUnionReturn = fn($param) : int|true => $param | 10;
+
+/* testUnionReturnTypeWithFalse */
+$arrowWithUnionReturn = fn($param) : string|FALSE => $param | 10;
+
 /* testTernary */
 $fn = fn($a) => $a ? /* testTernaryThen */ fn() : string => 'a' : /* testTernaryElse */ fn() : string => 'b';
 

tests/Core/Tokenizer/BackfillFnTokenTest.php

@@ -493,6 +493,15 @@ public static function dataKeywordReturnTypes()
             'static'   => [
                 'testMarker'  => '/* testStaticReturnType */',
             ],
+            'false'    => [
+                'testMarker'  => '/* testFalseReturnType */',
+            ],
+            'true'     => [
+                'testMarker'  => '/* testTrueReturnType */',
+            ],
+            'null'     => [
+                'testMarker'  => '/* testNullReturnType */',
+            ],
         ];
 
     }//end dataKeywordReturnTypes()
@@ -530,6 +539,38 @@ public function testUnionReturnType()
     }//end testUnionReturnType()
 
 
+    /**
+     * Test arrow function with a union return type.
+     *
+     * @covers PHP_CodeSniffer\Tokenizers\PHP::processAdditional
+     *
+     * @return void
+     */
+    public function testUnionReturnTypeWithTrue()
+    {
+        $token = $this->getTargetToken('/* testUnionReturnTypeWithTrue */', T_FN);
+        $this->backfillHelper($token);
+        $this->scopePositionTestHelper($token, 11, 18);
+
+    }//end testUnionReturnTypeWithTrue()
+
+
+    /**
+     * Test arrow function with a union return type.
+     *
+     * @covers PHP_CodeSniffer\Tokenizers\PHP::processAdditional
+     *
+     * @return void
+     */
+    public function testUnionReturnTypeWithFalse()
+    {
+        $token = $this->getTargetToken('/* testUnionReturnTypeWithFalse */', T_FN);
+        $this->backfillHelper($token);
+        $this->scopePositionTestHelper($token, 11, 18);
+
+    }//end testUnionReturnTypeWithFalse()
+
+
     /**
      * Test arrow functions used in ternary operators.
      *