Psalm unable to infer generic type when templated class implements a templated interface

[ocrampete16]

I've created a snippet to illustrate the issue: https://psalm.dev/r/557d8b895c

The first foreach block:

/** @var Collection<int, string> $foo */
$foo = new FooCollection(['foo']);
foreach ($foo as $bar) {
  $bar + 1;
}

results in Cannot perform a numeric operation with a non-numeric type string. So Psalm knows that $bar is a string. So far so good.

When I use the concrete class in the type hint (or leave the docblock out completely), Psalm is unable to infer that $bar is a string instead of mixed, even though it knows that it is of type FooCollection<int, string>:

/** @var FooCollection<int, string> $foo */
$foo = new FooCollection(['foo']);
foreach ($foo as $bar) {
  $bar + 1;
}

Running Psalm with the second foreach block gives me:

INFO: MixedAssignment - 45:18 - Cannot assign $bar to a mixed type

INFO: MixedOperand - 46:3 - Left operand cannot be mixed

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/557d8b895c

<?php

/**
 * @psalm-template TKey of array-key
 * @psalm-template T
 * @template-extends IteratorAggregate<TKey, T>
 */
interface Collection extends IteratorAggregate {}

/**
 * @psalm-template TKey of array-key
 * @psalm-template T
 * @template-implements Collection<TKey,T>
 */
class FooCollection implements Collection
{
  private $elements;
  
  /**
   * @psalm-param array<TKey,T> $elements
   */
  public function __construct(array $elements)
  {
    $this->elements = $elements;
  }
  
  public function getIterator()
  {
    return new ArrayIterator($this->elements);
  }
}

// Psalm knows that $bar is a string.
/** @var Collection<int, string> $foo */
$foo = new FooCollection(['foo']);
foreach ($foo as $bar) {
  $bar + 1;
}

// But when using the implementation in the type hint, $bar's type can't be inferred.
// Even though Psalm knows that $foo is of a "non-numeric type FooCollection<int, string>"
/** @var FooCollection<int, string> $foo */
$foo = new FooCollection(['foo']);
foreach ($foo as $bar) {
  $bar + 1;
}

Psalm output (using commit 352bd3f):

ERROR: InvalidOperand - 37:3 - Cannot perform a numeric operation with a non-numeric type string

INFO: MixedAssignment - 44:18 - Cannot assign $bar to a mixed type

INFO: MixedOperand - 45:3 - Left operand cannot be mixed

[muglug]

You can fix this by adding a return type to getIterator: https://psalm.dev/r/3f8b934c61

Still not sure if this is a bug, though

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/3f8b934c61

<?php

/**
 * @psalm-template TKey of array-key
 * @psalm-template T
 * @template-extends IteratorAggregate<TKey, T>
 */
interface Collection extends IteratorAggregate {}

/**
 * @psalm-template TKey of array-key
 * @psalm-template T
 * @template-implements Collection<TKey,T>
 */
class FooCollection implements Collection
{
  private $elements;
  
  /**
   * @psalm-param array<TKey,T> $elements
   */
  public function __construct(array $elements) {
    $this->elements = $elements;
  }
  
  /** @return ArrayIterator<TKey, T> */
  public function getIterator()
  {
    return new ArrayIterator($this->elements);
  }
}

/** @var Collection<int, string> $foo */
$foo = new FooCollection(['foo']);
foreach ($foo as $bar) {
  $bar + 1;
}

/** @var FooCollection<int, string> $foo */
$foo = new FooCollection(['foo']);
foreach ($foo as $bar) {
  $bar + 1;
}

Psalm output (using commit 352bd3f):

ERROR: InvalidOperand - 36:3 - Cannot perform a numeric operation with a non-numeric type string

ERROR: InvalidOperand - 42:3 - Cannot perform a numeric operation with a non-numeric type string

[ocrampete16]

Awesome, thanks for the swift fix!