Fix various unhandled cases where AT is part of a union type

Author: Girgias

Zend/tests/type_declarations/associated/associated_type_cannot_be_in_union4.phpt

@@ -0,0 +1,13 @@
+--TEST--
+Associated type cannot be in union (nullable type union with ?)
+--FILE--
+<?php
+
+interface I {
+    type T;
+    public function foo(?T $param): ?T;
+}
+
+?>
+--EXPECTF--
+Fatal error: Associated type cannot be part of a union type in %s on line %d

Zend/tests/type_declarations/associated/associated_type_cannot_be_in_union5.phpt

@@ -0,0 +1,13 @@
+--TEST--
+Associated type cannot be in union (forced allowed null)
+--FILE--
+<?php
+
+interface I {
+    type T;
+    public function foo(T $param = null): T;
+}
+
+?>
+--EXPECTF--
+Fatal error: Associated type cannot be part of a union type (implicitly nullable due to default null value) in %s on line %d

Zend/tests/type_declarations/associated/big_example.phpt

@@ -99,4 +99,4 @@ final class StringTable
 
 ?>
 --EXPECTF--
-Fatal error: Declaration of Sequence\StringTableSequence::next(): ?Sequence\StringTablePair must be compatible with Sequence\Sequence::next(): <Item : object|array|string|int|float|bool>|null in %s on line %d
+Fatal error: Associated type cannot be part of a union type in %s on line %d

Zend/zend_compile.c

@@ -7384,6 +7384,12 @@ static zend_type zend_compile_typename_ex(
 	if ((type_mask & MAY_BE_NULL) && is_marked_nullable) {
 		zend_error_noreturn(E_COMPILE_ERROR, "null cannot be marked as nullable");
 	}
+	if (ZEND_TYPE_IS_ASSOCIATED(type) && is_marked_nullable) {
+		zend_error_noreturn(E_COMPILE_ERROR, "Associated type cannot be part of a union type");
+	}
+	if (ZEND_TYPE_IS_ASSOCIATED(type) && force_allow_null) {
+		zend_error_noreturn(E_COMPILE_ERROR, "Associated type cannot be part of a union type (implicitly nullable due to default null value)");
+	}
 
 	if (force_allow_null && !is_marked_nullable && !(type_mask & MAY_BE_NULL)) {
 		*forced_allow_null = true;