Conditional return on interface template doesn't work

[thomasvargiu]

Using a conditional return, behaviour is different when defined in interface or in a class.

With class docblock:
https://psalm.dev/r/3cc2367e01

Using interface docblock:
https://psalm.dev/r/4484822d18

I would expect that conditional return works when defined in interfaces too.

Another thing I noticed is that (T is callable ? something : else) doesn't work.

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/3cc2367e01

<?php

/**
 * @template T
 */
interface A
{
    /**
     * @template V
     * @psalm-param V $value
     * @psalm-return (T is Closure ? A<V> : never-return)
     */
    public function test($value): A;
}

/**
 * @template T
 * @template-implements A<T>
 */
class B implements A
{
    /** @var T */
    private $value;
    /** @param T $value */
    public function __construct($value)
    {
        $this->value = $value;
    }
    
    /**
     * @template V
     * @psalm-param V $value
     * @psalm-return (T is Closure ? B<V> : never-return)
     */
    public function test($value): A
    {
        return new B($value);
    }
}

$bs = new B('foo');
$bf = new B(fn (string $a): int => strlen($a));

// this should return B<string>
/** @psalm-trace $result1 */
$result1 = $bf->test('foo');

// this should never-return
/** @psalm-trace $result2 */
$result2 = $bs->test('foo');

Psalm output (using commit 24c9702):

INFO: Trace - 46:1 - $result1: B<string(foo)>

ERROR: NoValue - 50:1 - This function or method call never returns output

https://psalm.dev/r/4484822d18

<?php

/**
 * @template T
 */
interface A
{
    /**
     * @template V
     * @psalm-param V $value
     * @psalm-return (T is Closure ? A<V> : never-return)
     */
    public function test($value): A;
}

/**
 * @template T
 * @template-implements A<T>
 */
class B implements A
{
    /** @var T */
    private $value;
    /** @param T $value */
    public function __construct($value)
    {
        $this->value = $value;
    }
    
    public function test($value): A
    {
        return new B($value);
    }
}

$bs = new B('foo');
$bf = new B(fn (string $a): int => strlen($a));

// this should return B<string>
/** @psalm-trace $result1 */
$result1 = $bf->test('foo');

// this should never-return
/** @psalm-trace $result2 */
$result2 = $bs->test('foo');

Psalm output (using commit 24c9702):

ERROR: NoValue - 41:1 - This function or method call never returns output

[thomasvargiu]

Probably related to #4524

[muglug]

Do you have an example of it breaking without never-return?

[muglug]

When both a docblock return type and a signature return type are given, Psalm performs a trivial check to see whether the docblock is more specific than the signature. If that check passes, the docblock is inheritable. Here that check isn't passing, so no inheritance of the docblock occurs (the implementing function specifies its own return type A.

Removing A from the signature allows this behaviour: https://psalm.dev/r/68e87c0681

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/68e87c0681

<?php

/**
 * @template T
 */
interface A
{
    /**
     * @template V
     * @psalm-param V $value
     * @psalm-return (T is Closure ? A<V> : never-return)
     */
    public function test($value);
}

/**
 * @template T
 * @template-implements A<T>
 */
class B implements A
{
    /** @var T */
    private $value;
    /** @param T $value */
    public function __construct($value)
    {
        $this->value = $value;
    }
    
    public function test($value)
    {
        return new B($value);
    }
}

$bs = new B('foo');
$bf = new B(fn (string $a): int => strlen($a));

// this should never-return
/** @psalm-trace $result2 */
$result2 = $bs->test('foo');

Psalm output (using commit a8d7248):

ERROR: NoValue - 41:1 - This function or method call never returns output

[muglug]

never-return wasn't intended for conditional return types, use them there at your own risk

[muglug]

Here's the trivial check if you want to add support for conditional return types, but it wouldn't be a simple change:

psalm/src/Psalm/Internal/Type/Comparator/UnionTypeComparator.php

Line 303 in 3803bbf

public static function isSimplyContainedBy(