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Add a centralized dependent_false type

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The main use case is to static_assert on it at the end of a
if constexpr / else if constexpr chain.

See P2593 for a discussion about why this is pretty much the
only "allowed" way of doing so, short of running into IFNDR.

I'm actually adding two versions: one for TTP and one for NTTP,
as Qt code uses both.

Apply it in QFlatMap.

Change-Id: Iaff97e350784683d0c3994020b1352d5188931d6
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
This commit is contained in:
Giuseppe D'Angelo 2022-12-05 12:59:10 +01:00
parent 3a449bbb69
commit 21425c05e0
2 changed files with 8 additions and 2 deletions
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7
src/corelib/global/qttypetraits.h
View File

@ -45,6 +45,13 @@ noexcept(std::conjunction_v<std::is_nothrow_move_constructible<T>,
return old; return old;
} }
namespace QtPrivate {
// helper to be used to trigger a "dependent static_assert(false)"
// (for instance, in a final `else` branch of a `if constexpr`.)
template <typename T> struct type_dependent_false : std::false_type {};
template <auto T> struct value_dependent_false : std::false_type {};
}
QT_END_NAMESPACE QT_END_NAMESPACE
#endif // QTTYPETRAITS_H #endif // QTTYPETRAITS_H

3
src/corelib/tools/qflatmap_p.h
View File

@ -848,7 +848,6 @@ public:
size_type remove_if(Predicate pred) size_type remove_if(Predicate pred)
{ {
const auto indirect_call_to_pred = [pred = std::move(pred)](iterator it) { const auto indirect_call_to_pred = [pred = std::move(pred)](iterator it) {
[[maybe_unused]] auto dependent_false = [](auto &&...) { return false; };
using Pair = decltype(*it); using Pair = decltype(*it);
using K = decltype(it.key()); using K = decltype(it.key());
using V = decltype(it.value()); using V = decltype(it.value());
@ -860,7 +859,7 @@ public:
} else if constexpr (std::is_invocable_v<P, K> && !std::is_invocable_v<P, Pair>) { } else if constexpr (std::is_invocable_v<P, K> && !std::is_invocable_v<P, Pair>) {
return pred(it.key()); return pred(it.key());
} else { } else {
static_assert(dependent_false(pred), static_assert(QtPrivate::type_dependent_false<Predicate>(),
"Don't know how to call the predicate.\n" "Don't know how to call the predicate.\n"
"Options:\n" "Options:\n"
"- pred(*it)\n" "- pred(*it)\n"