Add static return type

RFC: https://wiki.php.net/rfc/static_return_type

The "static" type is represented as MAY_BE_STATIC, rather than
a class type like "self" and "parent", as it has special
resolution semantics, and cannot be cached in the runtime cache.

Closes GH-5062.

Author: nikic

UPGRADING

@@ -367,6 +367,15 @@ PHP 8.0 UPGRADE NOTES
         class B extends A {
             public function method(...$everything) {}
         }
+  . "static" (as in "late static binding") can now be used as a return type:
+
+        class Test {
+            public function create(): static {
+                return new static();
+            }
+        }
+
+    RFC: https://wiki.php.net/rfc/static_return_type
   . It is now possible to fetch the class name of an object using
     `$object::class`. The result is the same as `get_class($object)`.
     RFC: https://wiki.php.net/rfc/class_name_literal_on_object

Zend/tests/type_declarations/static_type_in_final_class.phpt

@@ -0,0 +1,17 @@
+--TEST--
+While it may not be very useful, static is also permitted in final classes
+--FILE--
+<?php
+
+final class Test {
+    public static function create(): static {
+        return new static;
+    }
+}
+
+var_dump(Test::create());
+
+?>
+--EXPECT--
+object(Test)#1 (0) {
+}

Zend/tests/type_declarations/static_type_outside_class.phpt

@@ -0,0 +1,10 @@
+--TEST--
+Static type outside class generates compile error
+--FILE--
+<?php
+
+function test(): static {}
+
+?>
+--EXPECTF--
+Fatal error: Cannot use "static" when no class scope is active in %s on line %d

Zend/tests/type_declarations/static_type_param.phpt

@@ -0,0 +1,15 @@
+--TEST--
+Static type is not allowed in parameters
+--FILE--
+<?php
+
+// TODO: We could prohibit this case in the compiler instead.
+
+class Test {
+    public function test(static $param) {
+    }
+}
+
+?>
+--EXPECTF--
+Parse error: syntax error, unexpected 'static' (T_STATIC), expecting variable (T_VARIABLE) in %s on line %d

Zend/tests/type_declarations/static_type_property.phpt

@@ -0,0 +1,13 @@
+--TEST--
+Static type is not allowed in properties
+--FILE--
+<?php
+
+// Testing ?static here, to avoid ambiguity with static properties.
+class Test {
+    public ?static $foo;
+}
+
+?>
+--EXPECTF--
+Parse error: syntax error, unexpected 'static' (T_STATIC) in %s on line %d

Zend/tests/type_declarations/static_type_return.phpt

@@ -0,0 +1,91 @@
+--TEST--
+Static return type
+--FILE--
+<?php
+
+class A {
+    public function test1(): static {
+        return new static;
+    }
+    public function test2(): static {
+        return new self;
+    }
+    public function test3(): static {
+        return new static;
+    }
+    public function test4(): static|array {
+        return new self;
+    }
+}
+
+class B extends A {
+    public function test3(): static {
+        return new C;
+    }
+}
+
+class C extends B {}
+
+$a = new A;
+$b = new B;
+
+var_dump($a->test1());
+var_dump($b->test1());
+
+echo "\n";
+var_dump($a->test2());
+try {
+    var_dump($b->test2());
+} catch (TypeError $e) {
+    echo $e->getMessage(), "\n";
+}
+
+echo "\n";
+var_dump($a->test3());
+var_dump($b->test3());
+
+echo "\n";
+var_dump($a->test4());
+try {
+    var_dump($b->test4());
+} catch (TypeError $e) {
+    echo $e->getMessage(), "\n";
+}
+
+echo "\n";
+$test = function($x): static {
+    return $x;
+};
+
+try {
+    var_dump($test(new stdClass));
+} catch (TypeError $e) {
+    echo $e->getMessage(), "\n";
+}
+
+$test = $test->bindTo($a);
+var_dump($test($a));
+
+?>
+--EXPECT--
+object(A)#3 (0) {
+}
+object(B)#3 (0) {
+}
+
+object(A)#3 (0) {
+}
+Return value of A::test2() must be an instance of B, instance of A returned
+
+object(A)#3 (0) {
+}
+object(C)#3 (0) {
+}
+
+object(A)#3 (0) {
+}
+Return value of A::test4() must be of type B|array, instance of A returned
+
+Return value of {closure}() must be an instance of static, instance of stdClass returned
+object(A)#1 (0) {
+}

Zend/tests/type_declarations/static_type_trait.phpt

@@ -0,0 +1,38 @@
+--TEST--
+static type in trait
+--FILE--
+<?php
+
+trait T {
+    public function test($arg): static {
+        return $arg;
+    }
+}
+
+class C {
+    use T;
+}
+class P extends C {
+}
+
+$c = new C;
+$p = new P;
+var_dump($c->test($c));
+var_dump($c->test($p));
+var_dump($p->test($p));
+var_dump($p->test($c));
+
+?>
+--EXPECTF--
+object(C)#1 (0) {
+}
+object(P)#2 (0) {
+}
+object(P)#2 (0) {
+}
+
+Fatal error: Uncaught TypeError: Return value of C::test() must be an instance of P, instance of C returned in %s:%d
+Stack trace:
+#0 %s(%d): C->test(Object(C))
+#1 {main}
+  thrown in %s on line %d

Zend/tests/type_declarations/union_types/redundant_types/object_and_static.phpt

@@ -0,0 +1,12 @@
+--TEST--
+object and static are redundant
+--FILE--
+<?php
+
+class Test {
+    public function foo(): static|object {}
+}
+
+?>
+--EXPECTF--
+Fatal error: Type static|object contains both object and a class type, which is redundant in %s on line %d

Zend/tests/type_declarations/variance/static_variance_failure.phpt

@@ -0,0 +1,15 @@
+--TEST--
+Failure case for static variance: Replace static with self
+--FILE--
+<?php
+
+class A {
+    public function test(): static {}
+}
+class B extends A {
+    public function test(): self {}
+}
+
+?>
+--EXPECTF--
+Fatal error: Declaration of B::test(): B must be compatible with A::test(): static in %s on line %d

Zend/tests/type_declarations/variance/static_variance_success.phpt

@@ -0,0 +1,25 @@
+--TEST--
+Success cases for static variance
+--FILE--
+<?php
+
+class A {
+    public function test1(): self {}
+    public function test2(): B {}
+    public function test3(): object {}
+    public function test4(): X|Y|self {}
+    public function test5(): ?static {}
+}
+
+class B extends A {
+    public function test1(): static {}
+    public function test2(): static {}
+    public function test3(): static {}
+    public function test4(): X|Y|static {}
+    public function test5(): static {}
+}
+
+?>
+===DONE===
+--EXPECT--
+===DONE===