fix: display useful error message for invalid constructor return type

src/Mapper/Object/Exception/InvalidConstructorReturnType.php

@@ -5,16 +5,20 @@
 namespace CuyZ\Valinor\Mapper\Object\Exception;
 
 use CuyZ\Valinor\Definition\FunctionDefinition;
+use CuyZ\Valinor\Type\Types\UnresolvableType;
 use LogicException;
 
 /** @internal */
 final class InvalidConstructorReturnType extends LogicException
 {
     public function __construct(FunctionDefinition $function)
     {
-        parent::__construct(
-            "Invalid return type `{$function->returnType()->toString()}` for constructor `{$function->signature()}`, it must be a valid class name.",
-            1659446121
-        );
+        $returnType = $function->returnType();
+
+        $message = $returnType instanceof UnresolvableType
+            ? $returnType->getMessage()
+            : "Invalid return type `{$returnType->toString()}` for constructor `{$function->signature()}`, it must be a valid class name.";
+
+        parent::__construct($message, 1659446121);
     }
 }

tests/Integration/Mapping/ConstructorRegistrationMappingTest.php

@@ -16,6 +16,7 @@
 use CuyZ\Valinor\Tests\Fake\Mapper\Tree\Message\FakeErrorMessage;
 use CuyZ\Valinor\Tests\Integration\IntegrationTest;
 use CuyZ\Valinor\Tests\Integration\Mapping\Fixture\SimpleObject;
+use CuyZ\Valinor\Tests\Integration\Mapping\Fixture\SimpleObjectWithGeneric;
 use DateTime;
 use DateTimeImmutable;
 use DateTimeInterface;
@@ -568,6 +569,20 @@ public function test_invalid_constructor_return_type_throws_exception(): void
             ->map(stdClass::class, []);
     }
 
+    public function test_invalid_constructor_return_type_missing_generic_throws_exception(): void
+    {
+        $this->expectException(InvalidConstructorReturnType::class);
+        $this->expectExceptionCode(1659446121);
+        $this->expectExceptionMessageMatches('/The type `.*` for return type of method `.*` could not be resolved: No generic was assigned to the template\(s\) `T` for the class .*/');
+
+        (new MapperBuilder())
+            ->registerConstructor(
+                fn (): SimpleObjectWithGeneric => new SimpleObjectWithGeneric()
+            )
+            ->mapper()
+            ->map(stdClass::class, []);
+    }
+
     public function test_missing_constructor_class_type_parameter_throws_exception(): void
     {
         $this->expectException(MissingConstructorClassTypeParameter::class);

tests/Integration/Mapping/Fixture/SimpleObjectWithGeneric.php

@@ -0,0 +1,14 @@
+<?php
+
+declare(strict_types=1);
+
+namespace CuyZ\Valinor\Tests\Integration\Mapping\Fixture;
+
+/**
+ * @template T
+ */
+final class SimpleObjectWithGeneric
+{
+    /** @var T */
+    public $value;
+}