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@Codiferous is implementing WG21-P2280R4 in MSVC. With this Defect Report implemented, MSVC emits its (extremely annoying) warning C4127 "conditional expression is constant" for vector<bool>::max_size(), because we marked a local variable as const and that causes "trial evaluation" to happen. With WG21-P2280R4, _Ints_max is recognized as a compile-time constant, causing the if-statement below to emit the warning.
Unfortunately, the underlying vector's max_size() has to be implemented in terms of the allocator's max_size(), and there's no requirement that it be a compile-time constant. So we can't just boil this whole thing down to an if constexpr.
To avoid this, we can extract the condition into a const bool and add a comment why. C4127 has a special case to avoid warnings when a single variable is being tested.
(I am somewhat tempted to globally suppress C4127 in STL headers, but it hasn't been quite enough of a nuisance to resort to that... yet.)
I believe that extracting the condition to a const bool would avoid the warning, since there's a special case to avoid emitting it for if (variable), but that wouldn't allow us to upgrade the branch to if constexpr, because (AFAICT) there is no actual requirement that a user allocator has a constexpr max_size() (and pre-C++11 user allocators wouldn't).
Per WG21-P2280R4, IIUC we can determine whether the max_size returns an instance-unrelated constant and use if constexpr in some cases. But such determination is a bit arcane.
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@Codiferous is implementing WG21-P2280R4 in MSVC. With this Defect Report implemented, MSVC emits its (extremely annoying) warning C4127 "conditional expression is constant" for
vector<bool>::max_size(), because we marked a local variable asconstand that causes "trial evaluation" to happen. With WG21-P2280R4,_Ints_maxis recognized as a compile-time constant, causing theif-statement below to emit the warning.Unfortunately, the underlying
vector'smax_size()has to be implemented in terms of the allocator'smax_size(), and there's no requirement that it be a compile-time constant. So we can't just boil this whole thing down to anif constexpr.To avoid this, we can extract the condition into a
const booland add a comment why. C4127 has a special case to avoid warnings when a single variable is being tested.(I am somewhat tempted to globally suppress C4127 in STL headers, but it hasn't been quite enough of a nuisance to resort to that... yet.)