Try

Author: VincentLanglet

src/Type/Php/MbConvertEncodingFunctionReturnTypeExtension.php

@@ -52,12 +52,12 @@ public function getTypeFromFunctionCall(
 
 		$result = TypeCombinator::intersect($initialReturnType, $this->generalizeStringType($argType));
 		if ($result instanceof NeverType) {
-			return null;
+			$result = $initialReturnType;
 		}
 
 		if ($this->phpVersion->throwsValueErrorForInternalFunctions()) {
 			if (!isset($functionCall->getArgs()[2])) {
-				return $result;
+				return TypeCombinator::remove($result, new ConstantBooleanType(false));
 			}
 			$fromEncodingArgType = $scope->getType($functionCall->getArgs()[2]->value);
 
@@ -90,7 +90,7 @@ public function getTypeFromFunctionCall(
 			}
 
 			if (!$returnFalseIfCannotDetectEncoding) {
-				return $result;
+				return TypeCombinator::remove($result, new ConstantBooleanType(false));
 			}
 		}
 

tests/PHPStan/Analyser/nsrt/mb-convert-encoding-php8.php

@@ -20,15 +20,15 @@ function test_mb_convert_encoding(
 	string|array|bool $union,
 	int $int,
 ): void {
-	\PHPStan\Testing\assertType('array|string|false', mb_convert_encoding($mixed, 'UTF-8'));
+	\PHPStan\Testing\assertType('array|string', mb_convert_encoding($mixed, 'UTF-8'));
 	\PHPStan\Testing\assertType('string', mb_convert_encoding($constantString, 'UTF-8'));
 	\PHPStan\Testing\assertType('string', mb_convert_encoding($string, 'UTF-8'));
 	\PHPStan\Testing\assertType('array', mb_convert_encoding($mixedArray, 'UTF-8'));
 	\PHPStan\Testing\assertType('array{foo: string, bar: int, baz: string}', mb_convert_encoding($structuredArray, 'UTF-8'));
 	\PHPStan\Testing\assertType('list<string>', mb_convert_encoding($stringList, 'UTF-8'));
 	\PHPStan\Testing\assertType('list<int>', mb_convert_encoding($intList, 'UTF-8'));
 	\PHPStan\Testing\assertType('array{foo: string, bar: int, baz: string}|string|false', mb_convert_encoding($union, 'UTF-8'));
-	\PHPStan\Testing\assertType('array|string|false', mb_convert_encoding($int, 'UTF-8'));
+	\PHPStan\Testing\assertType('array|string', mb_convert_encoding($int, 'UTF-8'));
 
 	\PHPStan\Testing\assertType('string', mb_convert_encoding($string, 'UTF-8', 'FOO'));
 	\PHPStan\Testing\assertType('string|false', mb_convert_encoding($string, 'UTF-8', $string));