refactor(constructor): enhance constructor property validation with existing method checks

- Add detection of pre-existing __construct methods in class
- Throw CompileTimeAttributeError when constructor property conflicts with existing method
- Update test to use proper exception expectation pattern
- Add new test case for parameter order independence
- Create helper method to find declared constructors in class statements
- Improve error messaging with specific class and method information

phpunit/code/constructor-existing-reordered.php

@@ -0,0 +1,16 @@
+<?php
+
+class ReorderedConstructor
+{
+    #[Constructor]
+    private string $name;
+
+    #[Constructor]
+    private int $id;
+
+    public function __construct(int $id, string $name)
+    {
+        $this->id = $id;
+        $this->name = $name;
+    }
+}

phpunit/src/ClassTest.php

@@ -523,7 +523,20 @@ class ClassTest extends \BaseTest
 
     public function testConstructorRejectsExistingConstructor(): void
     {
-        $this->exec('Duplicate method `__construct`', 'constructor-existing.php');
+        $this->expectException(\TypePhp\Exception\SyntaxError::class);
+        $this->expectExceptionMessage(
+            'Constructor cannot generate InvalidConstructor::__construct(): method is already declared',
+        );
+        $this->compile('constructor-existing.php');
+    }
+
+    public function testConstructorRejectsExistingConstructorRegardlessOfParameterOrder(): void
+    {
+        $this->expectException(\TypePhp\Exception\SyntaxError::class);
+        $this->expectExceptionMessage(
+            'Constructor cannot generate ReorderedConstructor::__construct(): method is already declared',
+        );
+        $this->compile('constructor-existing-reordered.php');
     }
 
     public function testConstructorRejectsStaticProperties(): void

src/Transform/ConstructorLowering.php

@@ -33,6 +33,14 @@ final class ConstructorLowering
 
     public static function lowerClassLike(Stmt\Class_|Stmt\Trait_|Stmt\Enum_ $class): void
     {
+        $declaredConstructor = null;
+        foreach ($class->stmts as $stmt) {
+            if ($stmt instanceof Stmt\ClassMethod && $stmt->name->toLowerString() === '__construct') {
+                $declaredConstructor = $stmt;
+                break;
+            }
+        }
+
         $properties = [];
         $target = null;
         foreach ($class->stmts as $stmt) {
@@ -43,6 +51,17 @@ final class ConstructorLowering
                 throw new SyntaxError('Constructor properties can only be declared in classes');
             }
             $attribute = CompileTimeAttribute::find($stmt, 'Constructor');
+            if ($declaredConstructor !== null) {
+                $className = $class->name?->toString() ?? 'anonymous class';
+                throw new CompileTimeAttributeError(
+                    "Constructor cannot generate {$className}::__construct(): method is already declared",
+                    $stmt,
+                    'Constructor',
+                    $attribute ?? $stmt,
+                    null,
+                    $declaredConstructor,
+                );
+            }
             CompileTimeAttribute::consume($stmt, 'Constructor');
             $target ??= $stmt;
             foreach ($stmt->props as $property) {