Forbid implementing some interfaces

- `Throwable` can only be implemented when classes extend one of
  `Exception` or `Error`
- `UnitEnum` and `BackedEnum` cannot be implemented by user-defined
  classes

Refs vimeo/psalm#7722

config.xsd

@@ -272,6 +272,7 @@
             <xs:element name="InvalidFalsableReturnType" type="IssueHandlerType" minOccurs="0" />
             <xs:element name="InvalidFunctionCall" type="IssueHandlerType" minOccurs="0" />
             <xs:element name="InvalidGlobal" type="IssueHandlerType" minOccurs="0" />
+            <xs:element name="InvalidInterfaceImplementation" type="ClassIssueHandlerType" minOccurs="0" />
             <xs:element name="InvalidIterator" type="IssueHandlerType" minOccurs="0" />
             <xs:element name="InvalidLiteralArgument" type="ArgumentIssueHandlerType" minOccurs="0" />
             <xs:element name="InvalidMethodCall" type="IssueHandlerType" minOccurs="0" />

docs/running_psalm/error_levels.md

@@ -48,6 +48,7 @@ Level 5 and above allows a more non-verifiable code, and higher levels are even
  - [InvalidEnumMethod](issues/InvalidEnumMethod.md)
  - [InvalidExtendClass](issues/InvalidExtendClass.md)
  - [InvalidGlobal](issues/InvalidGlobal.md)
+ - [InvalidInterfaceImplementation](issues/InvalidInterfaceImplementation.md)
  - [InvalidParamDefault](issues/InvalidParamDefault.md)
  - [InvalidParent](issues/InvalidParent.md)
  - [InvalidPassByReference](issues/InvalidPassByReference.md)

docs/running_psalm/issues.md

@@ -75,6 +75,7 @@
  - [InvalidFalsableReturnType](issues/InvalidFalsableReturnType.md)
  - [InvalidFunctionCall](issues/InvalidFunctionCall.md)
  - [InvalidGlobal](issues/InvalidGlobal.md)
+ - [InvalidInterfaceImplementation](issues/InvalidInterfaceImplementation.md)
  - [InvalidIterator](issues/InvalidIterator.md)
  - [InvalidLiteralArgument](issues/InvalidLiteralArgument.md)
  - [InvalidMethodCall](issues/InvalidMethodCall.md)

docs/running_psalm/issues/InvalidInterfaceImplementation.md

@@ -0,0 +1,15 @@
+# InvalidInterfaceImplementation
+
+Emitted when trying to implement interface that cannot be implemented (e.g. `Throwable`, `UnitEnum`, `BackedEnum`).
+
+```php
+<?php
+
+class E implements UnitEnum 
+{
+    public static function cases(): array 
+    {
+        return []; 
+    }
+}
+```

src/Psalm/Internal/Analyzer/ClassAnalyzer.php

@@ -39,6 +39,7 @@ use Psalm\Issue\InaccessibleMethod;
 use Psalm\Issue\InternalClass;
 use Psalm\Issue\InvalidEnumCaseValue;
 use Psalm\Issue\InvalidExtendClass;
+use Psalm\Issue\InvalidInterfaceImplementation;
 use Psalm\Issue\InvalidTraversableImplementation;
 use Psalm\Issue\MethodSignatureMismatch;
 use Psalm\Issue\MismatchingDocblockPropertyType;
@@ -2114,6 +2115,35 @@ class ClassAnalyzer extends ClassLikeAnalyzer
                 );
             }
 
+            if ($fq_interface_name_lc === 'throwable'
+                && $codebase->analysis_php_version_id >= 7_00_00
+                && !$storage->abstract
+                && !isset($storage->parent_classes['exception'])
+                && !isset($storage->parent_classes['error'])
+            ) {
+                IssueBuffer::maybeAdd(
+                    new InvalidInterfaceImplementation(
+                        'Classes implementing Throwable should extend Exception or Error',
+                        $code_location,
+                        $fq_class_name,
+                    ),
+                );
+            }
+
+            if (($fq_interface_name_lc === 'unitenum'
+                    || $fq_interface_name_lc === 'backedenum')
+                && !$storage->is_enum
+                && $codebase->analysis_php_version_id >= 8_01_00
+            ) {
+                IssueBuffer::maybeAdd(
+                    new InvalidInterfaceImplementation(
+                        $fq_interface_name . ' cannot be implemented by classes',
+                        $code_location,
+                        $fq_class_name,
+                    ),
+                );
+            }
+
             if ($interface_storage->deprecated) {
                 IssueBuffer::maybeAdd(
                     new DeprecatedInterface(

src/Psalm/Issue/InvalidInterfaceImplementation.php

@@ -0,0 +1,9 @@
+<?php
+
+namespace Psalm\Issue;
+
+class InvalidInterfaceImplementation extends ClassIssue
+{
+    const ERROR_LEVEL = -1;
+    const SHORTCODE = 317;
+}

tests/ClassTest.php

@@ -1202,6 +1202,14 @@ class ClassTest extends TestCase
                 ',
                 'error_message' => 'MixedMethodCall',
             ],
+            'forbiddenThrowableImplementation' => [
+                'code' => '<?php
+                    class C implements Throwable {}
+                ',
+                'error_message' => 'InvalidInterfaceImplementation',
+                'ignored_issues' => [],
+                'php_version' => '7.0',
+            ],
         ];
     }
 }

tests/DocumentationTest.php

@@ -308,6 +308,7 @@ class DocumentationTest extends TestCase
                 case 'InvalidEnumMethod':
                 case 'NoEnumProperties':
                 case 'OverriddenFinalConstant':
+                case 'InvalidInterfaceImplementation':
                     $php_version = '8.1';
                     break;
             }

tests/EnumTest.php

@@ -765,6 +765,46 @@ class EnumTest extends TestCase
                 'ignored_issues' => [],
                 'php_version' => '8.1',
             ],
+            'forbiddenUnitEnumImplementation' => [
+                'code' => '<?php
+                    class Foo implements UnitEnum {
+                        /** @psalm-pure */
+                        public static function cases(): array
+                        {
+                            return [];
+                        }
+                    }
+                ',
+                'error_message' => 'InvalidInterfaceImplementation',
+                'ignored_issues' => [],
+                'php_version' => '8.1',
+            ],
+            'forbiddenBackedEnumImplementation' => [
+                'code' => '<?php
+                    class Foo implements BackedEnum {
+                        /** @psalm-pure */
+                        public static function cases(): array
+                        {
+                            return [];
+                        }
+
+                        /** @psalm-pure */
+                        public static function from(int|string $value): static
+                        {
+                            throw new Exception;
+                        }
+
+                        /** @psalm-pure */
+                        public static function tryFrom(int|string $value): ?static
+                        {
+                            return null;
+                        }
+                    }
+                ',
+                'error_message' => 'InvalidInterfaceImplementation',
+                'ignored_issues' => [],
+                'php_version' => '8.1',
+            ],
         ];
     }
 }