create a return type provider for mb_internal_encoding

src/Psalm/Internal/Provider/FunctionReturnTypeProvider.php

@@ -30,6 +30,7 @@
 use Psalm\Internal\Provider\ReturnTypeProvider\HexdecReturnTypeProvider;
 use Psalm\Internal\Provider\ReturnTypeProvider\InArrayReturnTypeProvider;
 use Psalm\Internal\Provider\ReturnTypeProvider\IteratorToArrayReturnTypeProvider;
+use Psalm\Internal\Provider\ReturnTypeProvider\MbInternalEncodingReturnTypeProvider;
 use Psalm\Internal\Provider\ReturnTypeProvider\MinMaxReturnTypeProvider;
 use Psalm\Internal\Provider\ReturnTypeProvider\MktimeReturnTypeProvider;
 use Psalm\Internal\Provider\ReturnTypeProvider\ParseUrlReturnTypeProvider;
@@ -97,6 +98,7 @@ public function __construct()
         $this->registerClass(RandReturnTypeProvider::class);
         $this->registerClass(InArrayReturnTypeProvider::class);
         $this->registerClass(RoundReturnTypeProvider::class);
+        $this->registerClass(MbInternalEncodingReturnTypeProvider::class);
     }
 
     /**

src/Psalm/Internal/Provider/ReturnTypeProvider/MbInternalEncodingReturnTypeProvider.php

@@ -0,0 +1,116 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Psalm\Internal\Provider\ReturnTypeProvider;
+
+use Psalm\Plugin\EventHandler\Event\FunctionReturnTypeProviderEvent;
+use Psalm\Plugin\EventHandler\FunctionReturnTypeProviderInterface;
+use Psalm\Type;
+use Psalm\Type\Atomic\TBool;
+use Psalm\Type\Atomic\TFalse;
+use Psalm\Type\Atomic\TNever;
+use Psalm\Type\Atomic\TNull;
+use Psalm\Type\Atomic\TString;
+use Psalm\Type\Atomic\TTrue;
+use Psalm\Type\Union;
+
+use function assert;
+use function count;
+
+/**
+ * @internal
+ */
+class MbInternalEncodingReturnTypeProvider implements FunctionReturnTypeProviderInterface
+{
+    /**
+     * @return array<lowercase-string>
+     */
+    public static function getFunctionIds(): array
+    {
+        return ['mb_internal_encoding'];
+    }
+
+    public static function getFunctionReturnType(FunctionReturnTypeProviderEvent $event): ?Union
+    {
+        $call_args = $event->getCallArgs();
+        if (count($call_args) === 0) {
+            return Type::getString();
+        }
+
+        $statements_source = $event->getStatementsSource();
+        $nodeTypeProvider = $statements_source->getNodeTypeProvider();
+        $codebase = $statements_source->getCodebase();
+
+        $first_arg_type = $nodeTypeProvider->getType($call_args[0]->value);
+        if ($first_arg_type === null) {
+            if ($codebase->analysis_php_version_id >= 8_00_00) {
+                return new Union([new TString(), new TTrue()]);
+            } else {
+                return new Union([new TString(), new TBool()]);
+            }
+        }
+
+        $has_stringable = false;
+        $has_tostring = false;
+        $has_string = false;
+        $has_null = false;
+        $has_unknown = false;
+
+        foreach ($first_arg_type->getAtomicTypes() as $atomic_type) {
+            if ($atomic_type instanceof Type\Atomic\TNamedObject
+                && $codebase->classlikes->classImplements($atomic_type->value, 'Stringable')
+            ) {
+                $has_stringable = true;
+                continue;
+            }
+
+            if ($atomic_type instanceof Type\Atomic\TObjectWithProperties
+                && isset($atomic_type->methods['__toString'])
+            ) {
+                $has_tostring = true;
+                continue;
+            }
+
+            if ($atomic_type instanceof TString) {
+                $has_string = true;
+                continue;
+            }
+
+            if ($atomic_type instanceof TNull) {
+                $has_null = true;
+                continue;
+            }
+
+            $has_unknown = true;
+        }
+
+        $list_return_atomics = [];
+        if ($has_string || $has_stringable || $has_tostring) {
+            if ($codebase->analysis_php_version_id >= 8_00_00) {
+                $list_return_atomics[] = new TTrue();
+            } else {
+                $list_return_atomics[] = new TBool();
+            }
+        }
+
+        if ($has_null) {
+            if ($codebase->analysis_php_version_id >= 8_00_00) {
+                $list_return_atomics[] = new TString();
+            } else {
+                $list_return_atomics[] = new TFalse();
+            }
+        }
+
+        if ($has_unknown) {
+            if ($codebase->analysis_php_version_id >= 8_00_00) {
+                $list_return_atomics[] = new TNever();
+            } else {
+                $list_return_atomics[] = new TNull();
+            }
+        }
+
+        assert($list_return_atomics !== []);
+        return new Union($list_return_atomics);
+    }
+}