minor #58094 Use Stringable whenever possible (fabpot)

This PR was merged into the 7.2 branch.

Discussion
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Use Stringable whenever possible

| Q             | A
| ------------- | ---
| Branch?       | 7.2
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Issues        | n/a
| License       | MIT

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Commits
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06b49968a5 Use Stringable whenever possible

Author: fabpot

Compiler/CheckTypeDeclarationsPass.php

@@ -274,7 +274,7 @@ private function checkType(Definition $checkedDefinition, mixed $value, \Reflect
             return;
         }
 
-        if ('string' === $type && method_exists($class, '__toString')) {
+        if ('string' === $type && $class instanceof \Stringable) {
             return;
         }