error types should be nothrow-movable

[ericniebler]

Issue by lucteo Sunday Nov 28, 2021 at 17:08 GMT Originally opened as https://github.com/NVIDIA/stdexec/issues/284

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Given:

• P2300R2 description of the set_error() CPO doesn't make any reference to whether set_error() can throw or not (9.4.2). The text implies that noexcept is forwarded to the actual implementation (most probably by the receiver objects)
• The receiver concept specification (9.4) defines the set_error() CPO as noexcept
• The let_error() specification allows the type E to throw in move construct
• The let_error() implementation example (9.6.4.5) implies that set_error(E) can throw (and a subsequent set_error() is called with the caught exception).

This seems confusing.

The only way that set_error() can throw inside the operation state is when the error type throws on move. If the E type can throw while being moved, then:

• as implementation show, we need a try/catch around the call to set_error()
• the sender construction can throw (as the error value is moved)
• the operation state can throw when constructed (i.e., when connect is called)

Again, the receiver connected to the operation state of let_error() doesn't allow set_error(E) to throw

Wouldn't it be better for the paper to specify that all error types are nothow-movable? The paper has this requirement in several places for other types. Adding this specification will make the implementation simpler and more coherent.

[ericniebler]

Comment by lucteo Sunday Nov 28, 2021 at 17:34 GMT

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Looking at the libunifex implementation (https://github.com/facebookexperimental/libunifex/blob/2547d87fb98a1fe70eccc170a8c91b144648517d/include/unifex/receiver_concepts.hpp), if the error type throws on move, then the program is ill formed. The set_error() CPO is defined as noexcept, it doesn't constrain the error type to not throw, and allows the move constructor to be called on the given error.

Thus, the inner call to tag_invoke may throw, without try/catch, inside a noexcept function.

[lewissbaker]

We should discuss about having this restriction in the set_error() CPO. i.e. that it mandates that the error type must be nothrow move-constructible.