`@psalm-assert-if-true !=''` widens `?class-string<X>` to `class-string<X>|non-empty-string`

[alies-dev]

Summary

When @psalm-assert-if-true !='' $value is applied to an input containing a class-string<X> atomic, Psalm adds a spurious non-empty-string atomic to the narrowed union instead of preserving the original class-string<X>. Since class-string<X> is already a subtype of non-empty-string, the assertion gives no new information, but the resulting union breaks downstream call sites like new $x() and $x::staticMethod() with InvalidStringClass.

Reproducer

<?php

/**
 * @param mixed $value
 * @psalm-assert-if-true !null $value
 * @psalm-assert-if-true !='' $value
 * @psalm-pure
 */
function isFilled($value): bool
{
    return $value !== null && $value !== '';
}

/**
 * Control: only !null, without !=''.
 *
 * @param mixed $value
 * @psalm-assert-if-true !null $value
 * @psalm-pure
 */
function isFilledControl($value): bool
{
    return $value !== null;
}

/**
 * @return ?class-string<Throwable>
 * @psalm-pure
 */
function getClass(): ?string { return null; }

function with_bad_assertion(): void
{
    $fqcn = getClass();
    if (isFilled($fqcn)) {
        /** @psalm-check-type-exact $fqcn = class-string<Throwable> */;
    }
}

function with_control_assertion(): void
{
    $fqcn = getClass();
    if (isFilledControl($fqcn)) {
        /** @psalm-check-type-exact $fqcn = class-string<Throwable> */;
    }
}

function downstream_invalid_string_class(): void
{
    /** @var ?class-string<DateTime> $cls */
    $cls = null;
    if (isFilled($cls)) {
        $cls::createFromFormat('Y-m-d', '2024-01-01');
    }
}

Expected

In both with_bad_assertion() and with_control_assertion(), $fqcn is class-string<Throwable> inside the true branch, and downstream_invalid_string_class() raises no issue (the static call is valid on a class-string<DateTime>).

Actual

ERROR: CheckType (at with_bad_assertion)
Checked variable $fqcn = class-string<Throwable> does not match $fqcn = class-string<Throwable>|non-empty-string

ERROR: InvalidStringClass (at downstream_invalid_string_class)
String cannot be used as a class

with_control_assertion() emits no CheckType, confirming that @psalm-assert-if-true !='' is the source. The !='' assertion is being applied to every atomic of the input union, and for class-string<X> it adds a non-empty-string atomic (even though class-string<X> is already a subtype of non-empty-string).

Impact

Real-world fallout in psalm/psalm-plugin-laravel#771: Laravel's filled() helper stub used this assertion pair to tighten ?string narrowing to non-empty-string. Benchmarking the plugin on Filament v4.8.3 vs v4.8.4 showed +35 new issues stemming from the assertion, including 14 InvalidStringClass on filled($classString = ...) && $classString::method() call sites. The assertion has since been reverted in the plugin, but the Psalm behavior should be fixed so !='' can be combined with class-string<X> inputs without widening.

Version

Psalm 7.0.0-beta19@7e751c06a756fa64dc4c759c09fe4a173afcb433

[psalm-github-bot]

Hey @alies-dev, can you reproduce the issue on https://psalm.dev? These will be used as phpunit tests when implementing the feature or fixing this bug.