This doesn't stop me from writing variations of this:

throw ((throw new Exception) ?? 1);

There are many such variations and you can't cover all of them with a simple AST check.

@nielsdos any thought how to prevent so?

But why do we need to prevent it at all? Compilers accept dumb programs, we can't possibly reject all of them (without also rejecting valid ones).

@nielsdos any thought how to prevent so?

I suppose you'd have to recursively check the AST subtree. But I agree with Ilija that trying to prevent this at compile time is likely not worth the hassle.

In any case it would also be a breaking change (legal PHP programs become illegal) and thus would require an RFC.

Should I close this then?

@xepozz You are free to create an RFC if you like. Such a check would be possible to implement by traversing the AST, or recording the fact that we're inside a throw expression and checking this flag in zend_compile_throw(). But I would not be in favor, as it doesn't achieve much. E.g. you can still write if (throw new Exception()) {}, 42 + throw new Exception, and so forth, none of which make sense. Such checks really belong in a static analyzer like PHPStan.

If you're not interested in creating an RFC, then yes, please close the issue.

by traversing the AST

I've thought about it, but it will degrade performance, won't it?

Also checking a flag in zend_compile_throw isn't a good idea for such case:

throw my_exc();

function my_exc() { throw new \Exception(); }

I've thought about it, but it will degrade performance, won't it?

Performance in the compiler is not particularly crucial, plus you can avoid the slowdown almost completely by storing a boolean flag in CG(context) and then checking whether this is set in zend_compile_throw().

Also checking a flag in zend_compile_throw isn't a good idea for such case:

This is not a concern. The call is not followed at compile-time.