Fixed bug #81582

If __toString() comes from a trait, implement Stringable during
inheritance.

NEWS

@@ -2,6 +2,10 @@ PHP                                                                        NEWS
 |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 ?? ??? 2021, PHP 8.0.14
 
+- Core:
+  . Fixed bug #81582 (Stringable not implicitly declared if __toString() came
+    from a trait). (Nikita)
+
 - GD:
   . Fixed bug #71316 (libpng warning from imagecreatefromstring). (cmb)
 

Zend/tests/stringable_trait.phpt

@@ -0,0 +1,20 @@
+--TEST--
+Bug #81582: Stringable not implicitly declared if __toString() came from a trait
+--FILE--
+<?php
+
+trait T {
+    public function __toString(): string {
+        return "ok";
+    }
+}
+
+class C {
+    use T;
+}
+
+var_dump(new C instanceof Stringable);
+
+?>
+--EXPECT--
+bool(true)

Zend/tests/stringable_trait_invalid.phpt

@@ -0,0 +1,20 @@
+--TEST--
+__toString() from trait with invalid return type
+--FILE--
+<?php
+
+trait T {
+    public function __toString(): int {
+        return "ok";
+    }
+}
+
+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

Zend/zend_inheritance.c

@@ -2485,6 +2485,18 @@ ZEND_API zend_result zend_do_link_class(zend_class_entry *ce, zend_string *lc_pa
 		zend_verify_abstract_class(ce);
 	}
 
+	/* Normally Stringable is added during compilation. However, if it is important from a trait,
+	 * we need to explicilty add the interface here. */
+	if (ce->__tostring && !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++;
+		ce->interfaces = perealloc(ce->interfaces,
+			sizeof(zend_class_entry *) * ce->num_interfaces, ce->type == ZEND_INTERNAL_CLASS);
+		ce->interfaces[ce->num_interfaces - 1] = zend_ce_stringable;
+		do_interface_implementation(ce, zend_ce_stringable);
+	}
+
 	zend_build_properties_info_table(ce);
 
 	if (!(ce->ce_flags & ZEND_ACC_UNRESOLVED_VARIANCE)) {