I found these snippets:

https://psalm.dev/r/6dbd395818

<?php

declare(strict_types=1);

/**
 * @return SplFixedArray<int>
 */
function getTraversable(): SplFixedArray
{
    return SplFixedArray::fromArray([1, 2, 3]);
}

/**
 * @return array<array-key, int>
 */
function getArray(): array
{
    return [1, 2, 3];
}

/**
 * @template A
 * @template B
 *
 * @param iterable<mixed, A> $lhs
 * @param iterable<mixed, B> $rhs
 * @return SplFixedArray<A|B>
 */
function mergeIterable(iterable $lhs, iterable $rhs): SplFixedArray
{
    return SplFixedArray::fromArray([...$lhs, ...$rhs]); 
}

/**
 * @template A
 * @template B
 *
 * @param array<array-key, A> $lhs
 * @param array<array-key, B> $rhs
 * @return SplFixedArray<A|B>
 */
function mergeArray(array $lhs, array $rhs): SplFixedArray
{
    return SplFixedArray::fromArray([...$lhs, ...$rhs]); 
}

/**
 * @template A
 * @template B
 *
 * @param Traversable<mixed, A> $lhs
 * @param Traversable<mixed, B> $rhs
 * @return SplFixedArray<A|B>
 */
function mergeTraversable(Traversable $lhs, Traversable $rhs): SplFixedArray
{
    return SplFixedArray::fromArray([...$lhs, ...$rhs]); 
}

// Works!
$_merged_traversable = mergeTraversable(getTraversable(), getTraversable());
/** @psalm-trace $_merged_traversable */;

// Works!
$_merged_array = mergeArray(getArray(), getArray());
/** @psalm-trace $_merged_array */;

// Don't :(
$_merged_iterable = mergeIterable(getTraversable(), getTraversable());
/** @psalm-trace $_merged_iterable */;

Psalm output (using commit 10ea05a):

INFO: Trace - 62:41 - $_merged_traversable: SplFixedArray<int>

INFO: Trace - 66:35 - $_merged_array: SplFixedArray<int>

INFO: Trace - 70:38 - $_merged_iterable: SplFixedArray<mixed>

https://psalm.dev/r/383c7f3677

<?php

declare(strict_types=1);

/**
 * @return SplFixedArray<int>
 */
function getTraversable(): SplFixedArray
{
    return SplFixedArray::fromArray([1, 2, 3]);
}

/**
 * @template A
 * @template B
 *
 * @param iterable<mixed, A> $lhs
 * @param iterable<mixed, B> $rhs
 * @return SplFixedArray<A|B>
 */
function mergeIterable(iterable $lhs, iterable $rhs): SplFixedArray
{
    return SplFixedArray::fromArray([...$lhs, ...$rhs]); 
}

// Expected SplFixedArray<int>, actual SplFixedArray<mixed>
$_merged_iterable = mergeIterable(getTraversable(), getTraversable());
/** @psalm-trace $_merged_iterable */;

Psalm output (using commit 10ea05a):

INFO: Trace - 28:38 - $_merged_iterable: SplFixedArray<mixed>

Invalid template instantiation for iterable #8124

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