Allow generators to have a real return type of mixed

Fixes an edge case overlooked in
php#5313

Author: TysonAndre

Zend/tests/return_types/generators001.phpt

@@ -14,10 +14,15 @@ function test3() : Traversable {
     yield 3;
 }
 
+function test4() : mixed {
+    yield 3;
+}
+
 var_dump(
     test1(),
     test2(),
-    test3()
+    test3(),
+    test4(),
 );
 --EXPECTF--
 object(Generator)#%d (%d) {
@@ -26,3 +31,5 @@ object(Generator)#%d (%d) {
 }
 object(Generator)#%d (%d) {
 }
+object(Generator)#%d (%d) {
+}

Zend/tests/return_types/generators002.phpt

@@ -6,4 +6,4 @@ function test1() : StdClass {
     yield 1;
 }
 --EXPECTF--
-Fatal error: Generators may only declare a return type containing Generator, Iterator, Traversable, or iterable, StdClass is not permitted in %s on line %d
+Fatal error: Generators may only declare a return type containing Generator, Iterator, Traversable, iterable, or mixed, StdClass is not permitted in %s on line %d

Zend/zend_compile.c

@@ -1263,7 +1263,8 @@ static void zend_mark_function_as_generator() /* {{{ */
 
 	if (CG(active_op_array)->fn_flags & ZEND_ACC_HAS_RETURN_TYPE) {
 		zend_type return_type = CG(active_op_array)->arg_info[-1].type;
-		zend_bool valid_type = (ZEND_TYPE_FULL_MASK(return_type) & MAY_BE_ITERABLE) != 0;
+		zend_bool valid_type = (ZEND_TYPE_FULL_MASK(return_type) & MAY_BE_ITERABLE) != 0 ||
+			ZEND_TYPE_FULL_MASK(return_type) == MAY_BE_ANY;
 		if (!valid_type) {
 			zend_type *single_type;
 			ZEND_TYPE_FOREACH(return_type, single_type) {
@@ -1279,7 +1280,7 @@ static void zend_mark_function_as_generator() /* {{{ */
 			zend_string *str = zend_type_to_string(return_type);
 			zend_error_noreturn(E_COMPILE_ERROR,
 				"Generators may only declare a return type containing " \
-				"Generator, Iterator, Traversable, or iterable, %s is not permitted",
+				"Generator, Iterator, Traversable, iterable, or mixed, %s is not permitted",
 				ZSTR_VAL(str));
 		}
 	}