[meta.unary.prop] [meta.rel] P/R for LWG3967 #8348
Conversation
This intends to eliminate all instances of "is known not to throw any exception" from the library clauses. "Is known" is too vague a condition (known to whom?); we ought to define it explicitly in terms of potentially-throwing expressions, i.e., we expect these traits to exactly match the result of the `noexcept` operator.
| variable definition for \tcode{is_constructible}, as defined below, is known not to | ||
| throw any exceptions\iref{expr.unary.noexcept}. | ||
| variable definition for \tcode{is_constructible}, as defined below, | ||
| contains no potentially-throwing expression\iref{except.spec}. |
There was a problem hiding this comment.
What does "contains" mean? The variable definition itself is not an expression.
There was a problem hiding this comment.
That's a fair question. I think what we're trying to say here is that below we have, textually, a variable definition T t(declval<Args>()...);, and we're asking whether that text produces any potentially-throwing expressions... but if the English word "contains" doesn't suffice to convey that, I don't immediately have any better suggestions.
I think https://eel.is/c++draft/except.spec#6 is the closest we come to prior art here. (It doesn't say "contains.")
There was a problem hiding this comment.
Well, the only expressions I'm seeing here are the argument expressions involving declval. The constructor invocation is not a (grammatical) expression, and neither are the default arguments (if any).
jensmaurer
added
the
changes requested
| \tcode{is_convertible_v<From, To>} is \tcode{true} and | ||
| the conversion, as defined by \tcode{is_convertible}, | ||
| is known not to throw any exceptions\iref{expr.unary.noexcept} & | ||
| is not a potentially-throwing expression\iref{except.spec}. & |
There was a problem hiding this comment.
We probably have another problem here, because a conversion is not an expression.
|
Perhaps we want to say "contains no potentially-throwing expression or potentially-throwing implicit function invocation in its immediate context". But CWG1844 is not yet resolved. |
3 participants
https://cplusplus.github.io/LWG/issue3967
This intends to eliminate all instances of "is known not to throw any exception" from the library clauses. "Is known" is too vague a condition (known to whom?); we ought to define it explicitly in terms of potentially-throwing expressions, i.e., we expect these traits to exactly match the result of the
noexceptoperator.