vimeo/psalm

Calling to `Map::union/intersect/diff()` etc. modifies args generic type

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#6,381 opened on Aug 31, 2021

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Help wantedbugtype reconciliation

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Description

interface A {
    /**
     * @param TReturn $value
     *
     * @return Map<TReturn, TReturn>
     *
     * @template TReturn
     */
    public static function fc(mixed $value) : Map
    {
    }
}

/** @var A $a */
$a;

$result = $a::fc(true);

Type of $result is correctly Ds\Map<true, true>.

Though, when I route $result through any mutation-free function on another map instance ((new Map)->union($result)), it changes to Ds\Map<mixed, mixed>. I don't see any reason for type change.

https://psalm.dev/r/4243e3b2a8
https://psalm.dev/r/123220a2c8

Contributor guide

Research direction
Trace how Psalm resolves the generic type of the result of Map::union when the argument is a map with a specific generic type. Check the type inference in the union method and see why it falls back to mixed.
Tech stack
php
Domain
tooling
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
PHPPsalm static analysis
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60