Fix false positive for a null return on mixed function

Inpsyde/Sniffs/CodeQuality/ReturnTypeDeclarationSniff.php

@@ -185,6 +185,12 @@ private function checkNonEmptyReturnTypes(
         array $returnInfo
     ): void {
 
+        if ($returnTypes === ['mixed']) {
+            $this->checkIsNotVoid($file, $position, $returnInfo);
+
+            return;
+        }
+
         if (($returnTypes === ['void']) || ($returnTypes === ['null'])) {
             $this->checkIsActualVoid($file, $position, $returnInfo, $returnTypes === ['null']);
 
@@ -196,6 +202,30 @@ private function checkNonEmptyReturnTypes(
             || $this->checkIncorrectVoid($file, $position, $returnTypes, $returnInfo);
     }
 
+    /**
+     * @param File $file
+     * @param int $position
+     * @param array $returnInfo
+     * @param bool $checkNull
+     * @return void
+     */
+    private function checkIsNotVoid(
+        File $file,
+        int $position,
+        array $returnInfo
+    ): void {
+
+        if ($returnInfo['void'] === 0) {
+            return;
+        }
+
+        $file->addError(
+            'Return type is declared non-void but void return statement(s) found',
+            $position,
+            'IncorrectVoidReturn'
+        );
+    }
+
     /**
      * @param File $file
      * @param int $position

tests/fixtures/return-type-declaration.php

@@ -3,6 +3,11 @@
 
 use Psr\Container\ContainerInterface as PsrContainer;
 
+function returnMixed(): mixed
+{
+    return null;
+}
+
 add_filter('x', function () {
     return '';
 });
@@ -218,6 +223,12 @@ function genMultiReturn(): \Generator
     return 2;
 }
 
+// @phpcsErrorCodeOnNextLine IncorrectVoidReturn
+function returnMixed2(): mixed
+{
+    return;
+}
+
 // @phpcsErrorCodeOnNextLine IncorrectVoidReturnType
 add_filter('x', function (): void {
     return '0';