fix: use strict mixed in StrictTypeSafetyChecker

Author: calebdw

rules/TypeDeclaration/NodeAnalyzer/StrictTypeSafetyChecker.php

@@ -18,6 +18,7 @@
 use PHPStan\Reflection\Php\PhpPropertyReflection;
 use PHPStan\Type\MixedType;
 use PHPStan\Type\NeverType;
+use PHPStan\Type\StrictMixedType;
 use PHPStan\Type\Type;
 use Rector\NodeTypeResolver\NodeTypeResolver;
 use Rector\NodeTypeResolver\PHPStan\ParametersAcceptorSelectorVariantsWrapper;
@@ -197,6 +198,11 @@ private function isPropertyAssignSafe(Assign $assign): bool
 
     private function isTypeSafeForStrictMode(Type $declaredType, Type $valueType): bool
     {
+        if ($valueType instanceof MixedType) {
+            /** @phpstan-ignore phpstanApi.constructor (need to be strict with mixed) */
+            $valueType = new StrictMixedType();
+        }
+
         return $declaredType->accepts($valueType, strictTypes: true)
             ->yes();
     }