Cover and document the matches operator throwing on an invalid UTF-8 subject

CHANGELOG

@@ -12,7 +12,7 @@
  * Fix the sandbox resolving `use` trait templates before checking that the `use` tag is allowed
  * Fix `html_attr` dropping `style` declarations whose value is `0`, `0.0` or `'0'`
  * Fix the `default` filter fallback emitting an undefined variable warning when it uses the null-safe operator
- * Fix the `matches` operator silently treating PCRE execution errors as non-matches
+ * Fix the `matches` operator silently treating PCRE execution errors as non-matches; it now throws a `RuntimeError`
  * Fix `TemplateWrapper::streamBlock()`, `TemplateWrapper::hasBlock()`, and `TemplateWrapper::getBlockNames()` omitting environment globals
  * Fix exceptions from dynamic parent expressions escaping without template context
  * Add the `BlockChain` class to compose blocks from multiple templates without using template internals

tests/Fixtures/expressions/matches_error_bad_utf8.test

@@ -0,0 +1,8 @@
+--TEST--
+Twig reports a PCRE failure when the "matches" subject is not valid UTF-8
+--TEMPLATE--
+{{ subject matches regexp }}
+--DATA--
+return ['subject' => "\xC3\x28abc", 'regexp' => '/^\p{L}+$/u'];
+--EXCEPTION--
+Twig\Error\RuntimeError: Regexp "/^\p{L}+$/u" passed to "matches" failed: Malformed UTF-8 characters, possibly incorrectly encoded in "index.twig" at line 2.