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Element-type narrowing by subtracting the null case

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

$arr = [$val] has type list{string|null}. After if ($arr === [null]), the else branch has excluded the only value where the element is null, so $arr narrows to list{string} and a call requiring list{string} should type-check. Analyzers diverge on whether they perform this equality-based element subtraction on the array shape. Source lead: Mago analyzer test array_reconcile (crates/analyzer/tests/cases), function test_null_string_subtraction. Mago narrows and accepts the guarded call; this file records every tool's verdict on the same construct.

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 Fail

No diagnostics reported.

Expectation diff
Line 41: Expected 1 error(s)
phpstan 2.2.11PHPStan - PHP Static Analysis Tool 2.2.11 Pass reported Lv.8+ 
regressions_array_element_null_subtraction.php:35: Parameter #1 $arr of function Conformance\Tests\RegressionsArrayElementNullSubtraction\takesStringList expects array{string}, array{string|null} given. [reported-from-level=8]
regressions_array_element_null_subtraction.php:41: Parameter #1 $arr of function Conformance\Tests\RegressionsArrayElementNullSubtraction\takesStringList expects array{string}, array{string|null} given. [reported-from-level=8]
With strict-rules
regressions_array_element_null_subtraction.php:35: Parameter #1 $arr of function Conformance\Tests\RegressionsArrayElementNullSubtraction\takesStringList expects array{string}, array{string|null} given.
regressions_array_element_null_subtraction.php:41: Parameter #1 $arr of function Conformance\Tests\RegressionsArrayElementNullSubtraction\takesStringList expects array{string}, array{string|null} given.
psalm 6.16.1Psalm 6.16.1@f1f5de594dc76faf8784e02d3dc4716c91c6f6ac
next: 7.0.0-beta19		 Pass 
regressions_array_element_null_subtraction.php:35: Argument 1 of Conformance\Tests\RegressionsArrayElementNullSubtraction\takesStringList expects list{string}, but parent type list{null|string} provided [ArgumentTypeCoercion]
regressions_array_element_null_subtraction.php:41: Argument 1 of Conformance\Tests\RegressionsArrayElementNullSubtraction\takesStringList expects list{string}, but parent type list{null|string} provided [ArgumentTypeCoercion]
mago 1.47.4mago 1.47.4 Pass 
regressions_array_element_null_subtraction.php:41: Possible argument type mismatch for argument #1 of `Conformance\Tests\RegressionsArrayElementNullSubtraction\takesStringList`: expected `list{string}`, but possibly received `list{null|string}`. [possibly-invalid-argument]
mir 0.72.1mir 0.72.1 Fail

No diagnostics reported.

Expectation diff
Line 41: Expected 1 error(s)
phpantom 0.10.0phpantom_lsp 0.10.0 Fail

No diagnostics reported.

Expectation diff
Line 41: Expected 1 error(s)
intelephense 1.18.5intelephense 1.18.5 Fail

No diagnostics reported.

Expectation diff
Line 41: Expected 1 error(s)
phpactor 2026.07.22.0Phpactor 2026.07.22.0 Fail

No diagnostics reported.

Expectation diff
Line 41: Expected 1 error(s)
phpy 0.2.0phpy 0.2.0 Fail

No diagnostics reported.

Expectation diff
Line 41: Expected 1 error(s)
qodana 262.9437.196Qodana 262.9437.196 Fail 
regressions_array_element_null_subtraction.php:20: Argument type does not match the declared [PhpDocSignatureInspection]
Expectation diff
Line 41: Expected 1 error(s)
noverify 0.5.5NoVerify, version 0.5.5: built on: 2025.04.22 14:36:46 OS: r-kuchinskas arm64 Commit: 4774b82f8adc53fbef38f95e97af4b953b4d1529 Fail 
regressions_array_element_null_subtraction.php:21: param $arr miss matched with phpdoc type <<list{string}>> [funcParamTypeMissMatch]
Expectation diff
Line 41: Expected 1 error(s)
steins 0.1.6steins 0.1.6 (2026-08-25 revision c13b4c7) Fail 
regressions_array_element_null_subtraction.php:21: parameter $arr of takesStringList() is an iterable with no value type — write `array<T>`, `T[]`, `list<T>` or an array shape [untyped.iterable-value]
Expectation diff
Line 41: Expected 1 error(s)
phpstan-strict 2.2.11PHPStan - PHP Static Analysis Tool 2.2.11 Pass 
regressions_array_element_null_subtraction.php:35: Parameter #1 $arr of function Conformance\Tests\RegressionsArrayElementNullSubtraction\takesStringList expects array{string}, array{string|null} given.
regressions_array_element_null_subtraction.php:41: Parameter #1 $arr of function Conformance\Tests\RegressionsArrayElementNullSubtraction\takesStringList expects array{string}, array{string|null} given.
psalm-next 7.0.0-beta19Psalm 7.0.0-beta19@7e751c06a756fa64dc4c759c09fe4a173afcb433 Pass 
regressions_array_element_null_subtraction.php:35: Argument 1 of Conformance\Tests\RegressionsArrayElementNullSubtraction\takesStringList expects list{string}, but parent type list{null|string} provided [ArgumentTypeCoercion]
regressions_array_element_null_subtraction.php:41: Argument 1 of Conformance\Tests\RegressionsArrayElementNullSubtraction\takesStringList expects list{string}, but parent type list{null|string} provided [ArgumentTypeCoercion]

Source

<?php

declare(strict_types=1);

namespace Conformance\Tests\RegressionsArrayElementNullSubtraction;

/**
 * Element-type narrowing by subtracting the null case.
 *
 * `$arr = [$val]` has type `list{string|null}`. After `if ($arr === [null])`, the else
 * branch has excluded the only value where the element is null, so `$arr` narrows to
 * `list{string}` and a call requiring `list{string}` should type-check. Analyzers diverge
 * on whether they perform this equality-based element subtraction on the array shape.
 *
 * Source lead: Mago analyzer test `array_reconcile` (crates/analyzer/tests/cases),
 * function `test_null_string_subtraction`. Mago narrows and accepts the guarded call;
 * this file records every tool's verdict on the same construct.
 */

/** @param list{string} $arr */ // E?[noise]: Qodana PhpDocSignatureInspection blames the @param for native array vs list{string}
function takesStringList(array $arr): void // E<noverify>: NoVerify cannot evaluate the list{string} shape param and flags the signature itself // E?[noise]: some tools flag native array vs list{string} on the signature
{
    printf("Count: %d\n", count($arr));
}

function caller(string|null $val): void
{
    $arr = [$val];

    if ($arr === [null]) {
        // the only null-element value is excluded here
        return;
    }

    takesStringList($arr); // E<phpstan>: does not subtract the null case via === [null], so $arr stays array{string|null} and reports argument.type // E<phpstan-strict>: same coercion under strict rules // E<psalm>: reports ArgumentTypeCoercion; element type stays string|null // E<pzoom>: same ArgumentTypeCoercion as Psalm. Mago narrows to list{string} and stays clean. Phan is silent on the unguarded call too, so it never checked
}

function unguarded(string|null $val): void
{
    $arr = [$val];
    takesStringList($arr); // E: list{string|null} is not list{string}
}