fix(compiler): enforce class constant override rules

phpunit/code/inheritance_error_const_final.php

@@ -0,0 +1,13 @@
+<?php
+
+class FinalConstantParent
+{
+    final public const VALUE = 1;
+}
+
+class FinalConstantChild extends FinalConstantParent
+{
+    public const VALUE = 2;
+}
+
+function main() {}

phpunit/code/inheritance_error_const_missing_type.php

@@ -0,0 +1,13 @@
+<?php
+
+class TypedConstantParent
+{
+    public const int VALUE = 1;
+}
+
+class UntypedConstantChild extends TypedConstantParent
+{
+    public const VALUE = 1;
+}
+
+function main() {}

phpunit/src/InheritanceErrorTest.php

@@ -168,6 +168,16 @@ class InheritanceErrorTest extends TestCase
         $this->exec('must be compatible', 'inheritance_error_const_type.php');
     }
 
+    public function testTypedConstantCannotBeOverriddenWithoutDeclaredType()
+    {
+        $this->exec('must be compatible', 'inheritance_error_const_missing_type.php');
+    }
+
+    public function testFinalConstantCannotBeOverridden()
+    {
+        $this->exec('Cannot override final constant', 'inheritance_error_const_final.php');
+    }
+
     public function testConstantVisibilityMismatch()
     {
         $this->exec('must be compatible', 'inheritance_error_const_visibility.php');

src/Parser/ClassConstantFetchTrait.php

@@ -32,15 +32,12 @@ trait ClassConstantFetchTrait
                 $class = $this->class;
             }
         } elseif ($class === 'parent') {
-            // `parent::` refers to the parent of the current class. Resolve it to
-            // the real parent class name and treat it like `self` for the purpose
-            // of constant/magic-class resolution.
-            $parentClass = $this->getParentClass($this->class);
-            if ($parentClass !== '' && $this->hasClass($parentClass)) {
-                $class = $this->getClass($parentClass)->name;
-            } else {
-                $class = $parentClass;
+            if (!$this->classDef || !$this->classDef->extends) {
+                $this->fatalError($expr, 'Cannot use "parent" outside a class or class does not extend any class');
             }
+            // extends is already fully resolved. Keep the leading slash so the
+            // current namespace is not applied again below.
+            $class = '\\' . $this->classDef->extends;
             $self = true;
         }
 
@@ -109,4 +106,3 @@ trait ClassConstantFetchTrait
     }
 
 }
-

src/Translator.php

@@ -3477,11 +3477,20 @@ CODE;
                     if ($parentConst->flags & Modifiers::PRIVATE) {
                         continue;
                     }
+                    if ($parentConst->flags & Modifiers::FINAL) {
+                        $this->fatalError($classStmt,
+                            "Cannot override final constant `{$parentClass}::{$name}`");
+                    }
                     // PHP only enforces type compatibility when the parent constant
                     // carries an explicit declared type. Overriding an untyped constant
                     // with a value of any type is permitted, so the type check is skipped
                     // in that case. Visibility is always enforced below.
                     if ($parentConst->declaredType !== null) {
+                        if ($childConst->declaredType === null) {
+                            $this->fatalError($classStmt,
+                                "Declaration of `{$className}::{$name}` must be compatible " .
+                                "with `{$parentClass}::{$name}`");
+                        }
                         // An untyped child constant whose value is an expression (e.g.
                         // `X = ParentClass::Y`) is inferred as a variant. Resolve its real
                         // type from the referenced constant so the compatibility check uses

tests/compiler/const/class-const-parent-cross-ns.phpt

@@ -0,0 +1,43 @@
+--TEST--
+parent class constants resolve across namespaces
+--FILE--
+<?php
+
+namespace Library {
+    class Base
+    {
+        public const TOKEN = 'base';
+    }
+}
+
+namespace Application {
+    class Sibling
+    {
+    }
+
+    class Child extends \Library\Base
+    {
+        public const PARENT_NAME = parent::class;
+        public const SIBLING_NAME = Sibling::class;
+
+        public static function dumpParent(): void
+        {
+            var_dump(parent::class, parent::TOKEN);
+        }
+    }
+}
+
+namespace {
+    function main(): void
+    {
+        var_dump(\Application\Child::PARENT_NAME);
+        var_dump(\Application\Child::SIBLING_NAME);
+        \Application\Child::dumpParent();
+    }
+}
+?>
+--EXPECT--
+string(12) "Library\Base"
+string(19) "Application\Sibling"
+string(12) "Library\Base"
+string(4) "base"