Don't implement Stringable on traits

Traits do not support interfaces, so we should not implement
Stringable on them.

Also check the __toString() return type in the same way other
magic methods do, otherwise we would now miss the check in the
trait case.

Author: nikic

Zend/tests/return_types/036.phpt

@@ -8,4 +8,4 @@ class Foo {
 }
 ?>
 --EXPECTF--
-Fatal error: Declaration of Foo::__toString(): bool must be compatible with Stringable::__toString(): string in %s on line %d
+Fatal error: Foo::__toString(): Return type must be string when declared in %s on line %d

Zend/tests/stringable_trait.phpt

@@ -8,13 +8,31 @@ trait T {
         return "ok";
     }
 }
+trait T2 {
+    use T;
+}
 
 class C {
     use T;
 }
+class C2 {
+    use T2;
+}
 
 var_dump(new C instanceof Stringable);
+var_dump(new C2 instanceof Stringable);
+
+// The traits themselves should not implement Stringable -- traits cannot implement interfaces.
+$rc = new ReflectionClass(T::class);
+var_dump($rc->getInterfaceNames());
+$rc = new ReflectionClass(T2::class);
+var_dump($rc->getInterfaceNames());
 
 ?>
 --EXPECT--
 bool(true)
+bool(true)
+array(0) {
+}
+array(0) {
+}

Zend/tests/stringable_trait_invalid.phpt

@@ -9,12 +9,6 @@ trait T {
     }
 }
 
-class C {
-    use T;
-}
-
-var_dump(new C instanceof Stringable);
-
 ?>
 --EXPECTF--
-Fatal error: Declaration of T::__toString(): int must be compatible with Stringable::__toString(): string in %s on line %d
+Fatal error: T::__toString(): Return type must be string when declared in %s on line %d

Zend/zend_API.c

@@ -2233,6 +2233,7 @@ ZEND_API void zend_check_magic_method_implementation(const zend_class_entry *ce,
 		zend_check_magic_method_args(0, ce, fptr, error_type);
 		zend_check_magic_method_non_static(ce, fptr, error_type);
 		zend_check_magic_method_public(ce, fptr, error_type);
+		zend_check_magic_method_return_type(ce, fptr, error_type, MAY_BE_STRING);
 	} else if (zend_string_equals_literal(lcname, ZEND_DEBUGINFO_FUNC_NAME)) {
 		zend_check_magic_method_args(0, ce, fptr, error_type);
 		zend_check_magic_method_non_static(ce, fptr, error_type);

Zend/zend_compile.c

@@ -6817,7 +6817,8 @@ zend_string *zend_begin_method_decl(zend_op_array *op_array, zend_string *name,
 	}
 
 	zend_add_magic_method(ce, (zend_function *) op_array, lcname);
-	if (zend_string_equals_literal(lcname, ZEND_TOSTRING_FUNC_NAME)) {
+	if (zend_string_equals_literal(lcname, ZEND_TOSTRING_FUNC_NAME)
+			&& !(ce->ce_flags & ZEND_ACC_TRAIT)) {
 		add_stringable_interface(ce);
 	}
 

Zend/zend_inheritance.c

@@ -2487,7 +2487,8 @@ ZEND_API zend_result zend_do_link_class(zend_class_entry *ce, zend_string *lc_pa
 
 	/* Normally Stringable is added during compilation. However, if it is imported from a trait,
 	 * we need to explicilty add the interface here. */
-	if (ce->__tostring && !zend_class_implements_interface(ce, zend_ce_stringable)) {
+	if (ce->__tostring && !(ce->ce_flags & ZEND_ACC_TRAIT)
+			&& !zend_class_implements_interface(ce, zend_ce_stringable)) {
 		ZEND_ASSERT(ce->__tostring->common.fn_flags & ZEND_ACC_TRAIT_CLONE);
 		ce->ce_flags |= ZEND_ACC_RESOLVED_INTERFACES;
 		ce->num_interfaces++;