vimeo/psalm

wrong template detection in composed instance

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#6,957 opened on Nov 22, 2021

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Description

Consider this snippet of code https://psalm.dev/r/6731c85f04

Given an object of type Iso<S, A>, its to method should be a function from S to A.

Passing an object $that of type Iso<A, C> as a parameter to a method of the ISO<S, A> object and checking the type of its to method, I would expect to get callable(A): C. Instead I get a callable(S): A. This is weird because $that doesn't know anything about the S template

Contributor guide

Research direction
Examine the template resolution logic in Psalm's type system, specifically how composed generics like Iso<S,A> and Iso<A,C> are handled when calling methods on instances. Look for where the template parameters are being incorrectly resolved to the outer instance's types.
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.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
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Active
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Clear
Prerequisites
PHP
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50