feat(errors): register UnhandledMatchError as a builtin Error subclass

src/types/checker/builtin_types/declarations.rs

@@ -72,6 +72,7 @@ pub(crate) fn inject_builtin_throwables(
         "TypeError",
         "ValueError",
         "ArithmeticError",
+        "UnhandledMatchError",
         "Exception",
         "RuntimeException",
         "JsonException",
@@ -246,6 +247,22 @@ pub(crate) fn inject_builtin_throwables(
             used_traits: Vec::new(),
         },
     );
+    class_map.insert(
+        "UnhandledMatchError".to_string(),
+        FlattenedClass {
+            name: "UnhandledMatchError".to_string(),
+            extends: Some("Error".to_string()),
+            implements: Vec::new(),
+            is_abstract: false,
+            is_final: false,
+            is_readonly_class: false,
+            properties: Vec::new(),
+            methods: Vec::new(),
+            attributes: Vec::new(),
+            constants: Vec::new(),
+            used_traits: Vec::new(),
+        },
+    );
 
     // Fiber: cooperative coroutine class. Methods are placeholders here — the
     // codegen intercepts every Fiber operation (`new Fiber(...)`, instance

src/types/checker/builtin_types/exception.rs

@@ -339,6 +339,7 @@ pub(crate) fn patch_builtin_exception_signatures(checker: &mut Checker) {
         "TypeError",
         "ValueError",
         "ArithmeticError",
+        "UnhandledMatchError",
         "Exception",
         "RuntimeException",
         "JsonException",

tests/codegen/exceptions.rs

@@ -624,3 +624,28 @@ try { echo $c->foo(); echo 'no'; } catch (Error $e) { echo 'err'; }
     );
     assert_eq!(out, "err");
 }
+
+/// UnhandledMatchError is a registered builtin Error subclass: newable with a message,
+/// throwable from a `match` default arm, catchable by its fully-qualified name, and its
+/// getMessage() round-trips — matching PHP 8's unhandled-match error type.
+#[test]
+fn test_unhandled_match_error_throw_and_catch() {
+    let out = compile_and_run(
+        r#"<?php
+function classify(int $n): string {
+    return match (true) {
+        $n < 0 => "neg",
+        $n === 0 => "zero",
+        default => throw new UnhandledMatchError("no arm for " . $n),
+    };
+}
+echo classify(-1), "|", classify(0), "|";
+try {
+    classify(5);
+} catch (\UnhandledMatchError $e) {
+    echo "caught:", $e->getMessage();
+}
+"#,
+    );
+    assert_eq!(out, "neg|zero|caught:no arm for 5");
+}