[intro.abstract] p6 A program may not have an observable behavior

[xmh0511]

Full name of submitter (unless configured in github; will be published with the issue): Jim X

[intro.abstract] p6 says:

A conforming implementation executing a well-formed program shall produce the observable behavior of the defined prefix of one of the possible executions of the corresponding instance of the abstract machine with the same program and the same input.

The emphasized wording sounds like it asserts that every possible execution does have observable behavior; however, this is not true. For example:

int main(){}

This is a well-formed program; however, any possible execution of the program doesn't have any observable behavior defined in [intro.abstract] p8. A real implementation plausibly does nothing for the program, which is reasonable to be considered as a conforming implementation.

Suggested Resolution:

A conforming implementation executing a well-formed program shall produces the observable behavior, if any, of the defined prefix of one of the possible executions of the corresponding instance of the abstract machine with the same program and the same input.

[languagelawyer]

Sequences can be empty. Prefixes can be empty.

[xmh0511]

Sequences can be empty. Prefixes can be empty.

defined prefix of an execution can comprise no operations; the element in the set is an operation, not observable behavior. Moreover, the phrase "the observable behavior of the defined prefix of one of the possible executions" seems to be unable to express vacuous truth, IIUC.

[eisenwave]

I think you're right that the observable behavior of a program can be empty. The fix seems quasi-editorial to me though; I don't think this needs to be a CWG issue.