Document that only plain string values are allowed in \Drupal\Tests\WebAssert::buildXPathQuery

I think you are talking about value of args. $args is supposed to be an array, but values of args can be an object (and shouldn't).

@dawehner yeah I agree, moving this fix to the BC layer makes sense to me if that is an option here.

Hm, I think we should close this as won't fix. Instead of polluting our buildXPathQuery() method we should remove the t() call in the actual test.

OK, documentation improvement sounds good.

WebAssert::buildXPathQuery() annotation already specifies that type of $xpath is string. Any other documentation change needed?

Yes, the exception message should say something like "Just pass in scalar values for $xpath and remove all t() calls from your test."

+++ b/core/tests/Drupal/Tests/WebAssert.php
@@ -320,7 +320,13 @@ public function buildXPathQuery($xpath, array $args = array()) {
+        // Cast Markup objects to a string.
+        if (method_exists($value, '__toString')) {
+          $value = (string) $value;
+        }

no, we should no cast the markup object. This patch should only update the exception message, right?

+++ b/core/tests/Drupal/Tests/WebAssert.php
@@ -320,7 +320,7 @@ public function buildXPathQuery($xpath, array $args = array()) {
     foreach ($args as $placeholder => $value) {
       if (is_object($value)) {
-        throw new \InvalidArgumentException('Just pass in scalar values.');
+        throw new \InvalidArgumentException('Just pass in scalar values for $xpath and remove all t() calls from your test.');

Ah sorry, of course we are iterating over $args and not $xpath. So it should be "Just pass in scalar values for $args and remove all t() calls from your test."

Committed 16ddd38 and pushed to 8.3.x. Thanks!

Less unnecessary t() calls in tests will be a nice thing.