Dead catch (catch.neverThrown) false positive around yield $key => $value - a yield with a key loses its implicit throw point #15249

Bug report

A yield is a point where a generator can be resumed with Generator::throw(), so PHPStan gives every yield an implicit throw point, and a surrounding catch is never reported as dead. That works for yield 1;, but not when the yield has a key. yield 'a' => 1; has no throw point at all:

/** @return \Generator<string, int, mixed, void> */
function withKey(): \Generator
{
	try {
		yield 'a' => 1;
	} catch (\RuntimeException $e) {
		echo $e->getMessage();
	}
}

/** @return \Generator<int, int, mixed, void> */
function withoutKey(): \Generator
{
	try {
		yield 1;
	} catch (\RuntimeException $e) {
		echo $e->getMessage();
	}
}

$generator = withKey();
$generator->current();
$generator->throw(new \RuntimeException('caught by withKey()'));

Actual output (level 4):

8: Dead catch - RuntimeException is never thrown in the try block.   [catch.neverThrown]

The catch in withKey() is reachable. Running the script prints caught by withKey(). The identical withoutKey() gets no error. The same false positive shows up for yield $k => 1 and $x = yield 'a' => 1. It doesn't show up for yield foo() => 1, because there the key expression brings its own implicit throw point from the function call, which hides the bug.

Cause: src/Analyser/ExprHandler/YieldHandler.php starts $throwPoints with InternalThrowPoint::createImplicit($scope, $expr). When the key is present, line 70 overwrites the array instead of merging into it:

$throwPoints = $keyResult->getThrowPoints();

The value branch just below does it right ($throwPoints = array_merge($throwPoints, $valueResult->getThrowPoints());), and so does the neighbouring $impurePoints line. The key's and the value's throw points are both kept. Only the yield's own implicit throw point is lost. The same $throwPoints also goes into VariableFlowBuilder::throws(), so anything that depends on the yield being a possible throw site (such as variable certainty in the catch block) is affected too.

Code snippet that reproduces the problem

https://phpstan.org/r/f5ac781b-381b-4b19-9f6b-1b25d80e9b2d

Expected output

No errors. yield $key => $value should keep the implicit throw point just like yield $value.

Suggested fix in YieldHandler::processExpr():

$throwPoints = array_merge($throwPoints, $keyResult->getThrowPoints());

Did PHPStan help you today? Did it make you happy in any way?

No response