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IncompatibleTypeParameters

Emitted when the constraints recorded for a class template parameter that could not be inferred at the construction site turn out to be incompatible with each other.

When a templated class is constructed, Psalm tracks each of its templates as a type variable: the constructor arguments and each later use add constraints, which are reconciled with each other once the surrounding function has been analyzed.

<?php

/** @template T of int */
class IntBox {
    public function __construct() {}

    /** @param T $item */
    public function add($item): void {}
}

function probe(): void {
    $box = new IntBox();
    $box->add("nope");
}

Here nothing binds T when IntBox is constructed, so T must be inferred from the later call to add() — but 'nope' cannot satisfy the template's of int constraint.

Why it’s bad

There is no type the template parameter could take that satisfies all of its recorded constraints, so the object is being used in contradiction with its own declaration.

How to fix

Pass values that satisfy the template's declared constraint:

<?php

/** @template T of int */
class IntBox {
    public function __construct() {}

    /** @param T $item */
    public function add($item): void {}
}

function probe(): void {
    $box = new IntBox();
    $box->add(42);
}