Implement basic variance support

This is a minimal variance implementation: It does not support any
cyclic type dependencies. Additionally the preloading requirements
are much more restrictive than necessary. Hopefully we can relax
these in the future.

Author: nikic

Zend/tests/bug62441.phpt

@@ -16,4 +16,4 @@ namespace ns {
 }
 ?>
 --EXPECTF--
-Fatal error: Declaration of ns\Foo::method(ns\stdClass $o) must be compatible with Iface::method(stdClass $o) in %s on line %d
+Fatal error: Could not check compatibility between ns\Foo::method(ns\stdClass $o) and Iface::method(stdClass $o), because class ns\stdClass is not available in %s on line %d

Zend/tests/return_types/008.phpt

@@ -14,6 +14,8 @@ class qux implements foo {
 }
 
 $qux = new qux();
-var_dump($qux->bar());
---EXPECTF--
-Fatal error: Declaration of qux::bar(): qux must be compatible with foo::bar(): foo in %s008.php on line 8
+echo get_class($qux->bar());
+
+?>
+--EXPECT--
+qux

Zend/tests/return_types/generators003.phpt

@@ -15,6 +15,8 @@ class SomeCollection implements Collection {
 }
 
 $some = new SomeCollection();
-var_dump($some->getIterator());
---EXPECTF--
-Fatal error: Declaration of SomeCollection::getIterator(): Generator must be compatible with Collection::getIterator(): Iterator in %sgenerators003.php on line 7
+echo get_class($some->getIterator());
+
+?>
+--EXPECT--
+Generator

Zend/tests/return_types/inheritance005.phpt

@@ -13,5 +13,9 @@ class Bar extends Foo {
         return new Bar;
     }
 }
---EXPECTF--
-Fatal error: Declaration of Bar::test(): Bar must be compatible with Foo::test(): Foo in %sinheritance005.php on line 9
+
+echo get_class(Bar::test());
+
+?>
+--EXPECT--
+Bar

Zend/tests/return_types/inheritance006.phpt

@@ -17,5 +17,9 @@ class Bar extends Foo {
         return new B;
     }
 }
---EXPECTF--
-Fatal error: Declaration of Bar::test(): B must be compatible with Foo::test(): A in %sinheritance006.php on line 11
+
+echo get_class(Bar::test());
+
+?>
+--EXPECT--
+B

Zend/tests/return_types/inheritance007.phpt

@@ -15,5 +15,9 @@ class Bar extends Foo {
         return new ArrayObject([1, 2]);
     }
 }
---EXPECTF--
-Fatal error: Declaration of Bar::test(): ArrayObject must be compatible with Foo::test(): Traversable in %sinheritance007.php on line 9
+
+echo get_class(Bar::test());
+
+?>
+--EXPECT--
+ArrayObject

Zend/tests/type_declarations/variance/class_order.phpt

@@ -0,0 +1,19 @@
+--TEST--
+Returns are covariant, but we don't allow the code due to class ordering
+--FILE--
+<?php
+
+class A {
+    public function method() : B {}
+}
+class B extends A {
+    public function method() : C {}
+}
+class C extends B {
+}
+
+new C;
+
+?>
+--EXPECTF--
+Fatal error: Could not check compatibility between B::method(): C and A::method(): B, because class C is not available in %s on line %d

Zend/tests/type_declarations/variance/class_order_autoload.phpt

@@ -0,0 +1,25 @@
+--TEST--
+Returns are covariant, but we don't allow the code due to class ordering (autoload variation)
+--FILE--
+<?php
+
+spl_autoload_register(function($class) {
+    if ($class === 'A') {
+        class A {
+            public function method() : B {}
+        }
+    } else if ($class == 'B') {
+        class B extends A {
+            public function method() : C {}
+        }
+    } else {
+        class C extends B {
+        }
+    }
+});
+
+$c = new C;
+
+?>
+--EXPECTF--
+Fatal error: Could not check compatibility between B::method(): C and A::method(): B, because class C is not available in %s on line %d

Zend/tests/type_declarations/variance/enum_forward_compat.phpt

@@ -0,0 +1,26 @@
+--TEST--
+Forward compatibility with types that look like classes but aren't
+--FILE--
+<?php
+
+spl_autoload_register(function($class) {
+    var_dump($class);
+    if ($class === 'X') {
+        class X {}
+    } else {
+        class Y {}
+    }
+});
+
+class A {
+    public function method(X $param) : object {}
+}
+
+class B extends A {
+    public function method(object $param) : Y {}
+}
+
+?>
+--EXPECT--
+string(1) "X"
+string(1) "Y"

Zend/tests/type_declarations/variance/object_variance.phpt

@@ -0,0 +1,22 @@
+--TEST--
+Testing object's variance in inheritance
+--FILE--
+<?php
+
+interface I1 {
+  function method1(I1 $o): object;
+}
+interface I2 extends I1 {
+  function method1(object $o): I1;
+}
+final class C1 implements I2 {
+  function method1($o = null): self {
+    return $this;
+  }
+}
+
+$o = new C1();
+echo get_class($o->method1());
+?>
+--EXPECT--
+C1

Zend/tests/type_declarations/variance/parent_in_class.phpt

@@ -0,0 +1,45 @@
+--TEST--
+Use of parent inside a class that has / has no parent
+--FILE--
+<?php
+
+// Illegal: A::parent is ill-defined
+class A {
+    public function method(parent $x) {}
+}
+class B extends A {
+    public function method(parent $x) {}
+}
+
+// Legal: A2::parent == P2
+class P2 {}
+class A2 extends P2 {
+    public function method(parent $x) {}
+}
+class B2 extends A2 {
+    public function method(P2 $x) {}
+}
+
+// Legal: B3::parent == A3 is subclass of A3::parent == P3 in covariant position
+class P3 {}
+class A3 extends P3 {
+    public function method($x): parent {}
+}
+class B3 extends A3 {
+    public function method($x): parent {}
+}
+
+// Illegal: B4::parent == A4 is subclass of A4::parent == P4 in contravariant position
+class P4 {}
+class A4 extends P4 {
+    public function method(parent $x) {}
+}
+class B4 extends A4 {
+    public function method(parent $x) {}
+}
+
+?>
+--EXPECTF--
+Warning: Declaration of B4::method(A4 $x) should be compatible with A4::method(P4 $x) in %s on line %d
+
+Warning: Could not check compatibility between B::method(A $x) and A::method(parent $x), because class parent is not available in %s on line %d