Repository navigation
CWG2668 [expr.await] await-expression is forbidden to appear in the lambda-expression #195
Description
Full name of submitter (unless configured in github; will be published with the issue): Jim X
[expr.await] p2 says
An await-expression shall appear only in a potentially-evaluated expression within the compound-statement of a function-body outside of a handler ([except.pre]).
Consider this example
auto f = []()->Task{
co_await Awaitable{};
}Grammarly, the await-expression appears in the compound-statement of a lambda-expression rather than the compound-statement of a function-body, even if [expr.prim.lambda.closure] p13
The lambda-expression's compound-statement yields the function-body ([dcl.fct.def]) of the function call operator, but it is not within the scope of the closure type.
Anyway, they are not equivalent in grammar.
Suggested resolution
An await-expression shall appear only in a potentially-evaluated expression within:
- the compound-statement of a function-body outside of a handler ([except.pre]), or
- the compound-statement of a lambda-expression
Activity
jensmaurer commentedon Dec 17, 2022
"outside of a handler" should also apply to the lambda-expression case.
jensmaurer commentedon Dec 17, 2022
CWG2668
[-][expr.await] await-expression is forbidden to appear in the lambda-expression[/-][+]CWG2668 [expr.await] await-expression is forbidden to appear in the lambda-expression[/+]xmh0511 commentedon Dec 18, 2022
Where does the handler appear in a lambda-expression? The original wording says
IIUC, it means that the await-expression can only appear within the compound-statement of a function-body, and shouldn't appear in the handler if the function-body is a function-try-block. In other words, the following example is forbidden
Conversely, the below code is well-formed
Since lambda-expression itself does not have a handler component, I don't think "outside of a handler" should also apply to the lambda-expression case.
Or, does the original wording intend to mean both cases are ill-formed?
jensmaurer commentedon Dec 18, 2022
That's my reading. Both cases are "handlers", I think.
xmh0511 commentedon Dec 18, 2022
Thanks, you're right. I tested the code in GCC and Clang, and they all agree that the await-expression cannot appear within a handler, that is, "or..." is the proper reading.