bool" requirement in conjunction and disjunctionSection: 3.3.3 [fund.ts.v2::meta.logical] Status: TS Submitter: Tim Song Opened: 2016-01-18 Last modified: 2017-07-30
Priority: 3
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Discussion:
Addresses: fund.ts.v2
The specification of conjunction and disjunction in 3.3.3 [fund.ts.v2::meta.logical] p2 and p5 requires
Bi::value to be convertible to bool, but nothing in the specification of the actual behavior of the
templates, which instead uses the expressions Bi::value == false and Bi::value != false instead,
actually requires this conversion.
Bi::value directly to std::conditional,
like the sample implementation in P0013R1:
template<class B1, class B2>
struct and_<B1, B2> : conditional_t<B1::value, B2, B1> { };
then it's insufficient in at least two ways:
Nothing in the specification requires the result of comparing Bi::value with false to be consistent
with the result of the implicit conversion. This is similar to LWG 2114, though I don't think the
BooleanTestable requirements in that issue's P/R covers Bi::value == false and Bi::value != false.
More importantly, the above implementation is ill-formed for, e.g.,
std::conjunction<std::integral_constant<int, 2>, std::integral_constant<int, 4>>, because converting 2
to bool is a narrowing conversion that is not allowed for non-type template arguments (see 7.7 [expr.const]/4).
(Note that GCC currently doesn't diagnose this error at all, and Clang doesn't diagnose it inside system headers.) It's not clear
whether such constructs are intended to be supported, but if they are not, the current wording doesn't prohibit it.
[2016-11-08, Issaquah]
Adopted during NB comment resolution
Proposed resolution:
The resolution for this issue was combined with the resolution for LWG 2568, so 2568 resolves this issue here as well.