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Narrowing an object union by a discriminating property

Group: Cross-analyzer regressions · Category: regression · File: regressions_object_property_discriminant_narrowing.php

StrBox::$v is string and IntBox::$v is int, so is_string($b->v) holds only for StrBox. In that branch $b is StrBox and return $b satisfies the : StrBox return type. Analyzers diverge on whether they narrow the *object* union from a check on a discriminating property. PHPStan narrows array-shape discriminated unions ($a['tag'] === true) but not the object equivalent (is_string($b->v) / $b->tag === true), so it keeps StrBox|IntBox here and reports a false-positive return.type. Source lead: Mago analyzer test issue_1093 (test_property_narrowing).

Analyzer results

AnalyzerVersionResultDiagnostics
phan 6.0.7Phan 6.0.7 php-ast version 1.1.3 PHP version used to run Phan: 8.5.9 Pass

No diagnostics reported.

phpstan 2.2.8PHPStan - PHP Static Analysis Tool 2.2.8 Pass reported Lv.7+ 
regressions_object_property_discriminant_narrowing.php:35: Function Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\pick() should return Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\StrBox but returns Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\IntBox|Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\StrBox. [identifier=return.type] [reported-from-level=7]
With strict-rules
regressions_object_property_discriminant_narrowing.php:35: Function Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\pick() should return Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\StrBox but returns Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\IntBox|Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\StrBox. [identifier=return.type]
psalm 6.16.1Psalm 6.16.1@f1f5de594dc76faf8784e02d3dc4716c91c6f6ac
next: 7.0.0-beta19		 Pass (pzoom≠) 
regressions_object_property_discriminant_narrowing.php:32: The declared return type 'Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\StrBox' for Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\pick is incorrect, got 'Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\IntBox|Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\StrBox' [InvalidReturnType]
regressions_object_property_discriminant_narrowing.php:35: The inferred type 'Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\IntBox|Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\StrBox' does not match the declared return type 'Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\StrBox' for Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\pick [InvalidReturnStatement]
pzoom (Psalm port)
regressions_object_property_discriminant_narrowing.php:35: The type Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\IntBox|Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\StrBox does not match the declared return type Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\StrBox [InvalidReturnStatement]
mago 1.46.0mago 1.46.0 Pass

No diagnostics reported.

mir 0.70.1mir 0.70.1 Pass

No diagnostics reported.

phpantom 0.9.0phpantom_lsp 0.9.0 Pass

No diagnostics reported.

intelephense 1.18.5intelephense 1.18.5 Fail 
regressions_object_property_discriminant_narrowing.php:35: Expected type 'Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\StrBox'. Found 'Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\StrBox|Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\IntBox'. [P1006]
Expectation diff
Line 35: Unexpected errors ["Expected type 'Conformance\\Tests\\RegressionsObjectPropertyDiscriminantNarrowing\\StrBox'. Found 'Conformance\\Tests\\RegressionsObjectPropertyDiscriminantNarrowing\\StrBox|Conformance\\Tests\\RegressionsObjectPropertyDiscriminantNarrowing\\IntBox'. [P1006]"]
phpy 0.2.0phpy 0.2.0 Pass

No diagnostics reported.

qodana 262.8665.325Qodana 262.8665.325 Pass

No diagnostics reported.

noverify 0.5.5NoVerify, version 0.5.5: built on: 2025.04.22 14:36:46 OS: r-kuchinskas arm64 Commit: 4774b82f8adc53fbef38f95e97af4b953b4d1529 Pass

No diagnostics reported.

steins 0.1.5steins 0.1.5 (2026-08-12 revision 39b4cc1) Pass

No diagnostics reported.

phpstan-strict 2.2.8PHPStan - PHP Static Analysis Tool 2.2.8 Pass 
regressions_object_property_discriminant_narrowing.php:35: Function Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\pick() should return Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\StrBox but returns Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\IntBox|Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\StrBox. [identifier=return.type]
psalm-next 7.0.0-beta19Psalm 7.0.0-beta19@7e751c06a756fa64dc4c759c09fe4a173afcb433 Pass 
regressions_object_property_discriminant_narrowing.php:32: The declared return type 'Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\StrBox' for Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\pick is incorrect, got 'Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\IntBox|Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\StrBox' [InvalidReturnType]
regressions_object_property_discriminant_narrowing.php:35: The inferred type 'Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\IntBox|Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\StrBox' does not match the declared return type 'Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\StrBox' for Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing\pick [InvalidReturnStatement]

Source

<?php

declare(strict_types=1);

namespace Conformance\Tests\RegressionsObjectPropertyDiscriminantNarrowing;

/**
 * Narrowing an object union by a discriminating property.
 *
 * `StrBox::$v` is `string` and `IntBox::$v` is `int`, so `is_string($b->v)` holds only for
 * `StrBox`. In that branch `$b` is `StrBox` and `return $b` satisfies the `: StrBox` return
 * type. Analyzers diverge on whether they narrow the *object* union from a check on a
 * discriminating property.
 *
 * PHPStan narrows array-shape discriminated unions (`$a['tag'] === true`) but not the object
 * equivalent (`is_string($b->v)` / `$b->tag === true`), so it keeps `StrBox|IntBox` here and
 * reports a false-positive return.type.
 *
 * Source lead: Mago analyzer test `issue_1093` (`test_property_narrowing`).
 */

final class StrBox
{
    public string $v = '';
}

final class IntBox
{
    public int $v = 0;
}

function pick(StrBox|IntBox $b): StrBox // E<psalm>: declares the return invalid because $b is not narrowed to StrBox (InvalidReturnType)
{
    if (is_string($b->v)) {
        return $b; // E<phpstan>: does not narrow the object union from is_string($b->v), so keeps StrBox|IntBox and reports return.type // E<phpstan-strict>: same // E<psalm>: InvalidReturnStatement. Mago and Phan narrow to StrBox and stay clean
    }

    throw new \LogicException('not a StrBox');
}