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Code Coverage Summary

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@tbradsha like this, for example?

/**
 * Filters foo.
 *
 * @param string|mixed $foo Foo.
 * @return string Foo.
 */
function filter_foo( $foo, int $post_id ): string {
	if ( ! is_string( $foo ) ) {
		// Because plugins do bad things.
		$foo = '';
	}

	 // Filtering logic goes here.

	 return $foo;
}
add_filter( 'foo', 'filter_foo', 10, 2 );

like this, for example?

Yep, precisely. Since the widget_text filter is documented to pass a string, we should:

• check for a string type, but allow for any type, since
  1. the content could be filtered in third-party code to a bad type prior to getting to the function
  2. there could be a third-party apply_filter that passes a bad type
• return a string type (be it an empty or fallback string), since that prevents similar errors for the next function that's hooked into the filter.

I was torn between passing through malformed vs returning an empty string instead. Hearing y'all, I'll update the code and tests to return '' vs passthrough.