https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110871
Bug ID: 110871
Summary: coroutine precondition should be evaluated before the
initial suspend
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: akrzemi1 at gmail dot com
Target Milestone: ---
Created attachment 55672
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55672&action=edit
Sample program illustrating the bug
When I declare a precondition on a coroutine, it is not evaluated when the
corresponding promise type performs the initial suspend.
While "C++20 contracts" or the "Contracts MVP"
(https://isocpp.org/files/papers/P2388R4.html) never mention how this should
work, the intuition dictates that the preconditions protect against the usage
of the incorrect combination of function parameters.
Function parameters of a coroutine may be used before the initial suspend to
initialize the promise object (when it provides a variadic constructor). The
coroutine precondition should be evaluated before this initialization,
therefore before the initial suspend.
Also, this follows from the rule of thumb that implementations should have
freedom to check the precondition either in the caller or in the callee and
either choice should not affect the program semantics.
The following Compiler Explorer example illustrtes the issue. "Disaster!"
should never be printed:
https://godbolt.org/z/fxMjT4vfx
