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[libc++] span: Fix incorrect static asserts

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The static asserts in span<T, N>::front() and span<T, N>::back() are
incorrect as they may be triggered from valid code due to evaluation
of a never taken branch:

    span<int, 0> foo;
    if (!foo.empty()) {
        auto x = foo.front();
    }

The problem is that the branch is always evaluated by the compiler,
creating invalid compile errors for span<T, 0>.

Thanks to Michael Schellenberger Costa for the patch.

Differential Revision: https://reviews.llvm.org/D71995
This commit is contained in:
Louis Dionne 2020-02-14 14:31:15 +01:00
parent 84240e0db8
commit 0a0e0afaa0
3 changed files with 20 additions and 3 deletions
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4
libcxx/include/span
View File

@ -307,13 +307,13 @@ public:
_LIBCPP_INLINE_VISIBILITY constexpr reference front() const noexcept
{
static_assert(_Extent > 0, "span<T,N>[].front() on empty span");
_LIBCPP_ASSERT(!empty(), "span<T, N>::front() on empty span");
return __data[0];
}
_LIBCPP_INLINE_VISIBILITY constexpr reference back() const noexcept
{
static_assert(_Extent > 0, "span<T,N>[].back() on empty span");
_LIBCPP_ASSERT(!empty(), "span<T, N>::back() on empty span");
return __data[size()-1];
}

10
libcxx/test/std/containers/views/span.elem/back.pass.cpp
View File

@ -30,7 +30,6 @@ constexpr bool testConstexprSpan(Span sp)
return std::addressof(sp.back()) == sp.data() + sp.size() - 1;
}
template <typename Span>
void testRuntimeSpan(Span sp)
{
@ -38,6 +37,12 @@ void testRuntimeSpan(Span sp)
assert(std::addressof(sp.back()) == sp.data() + sp.size() - 1);
}
template <typename Span>
void testEmptySpan(Span sp)
{
if (!sp.empty())
[[maybe_unused]] auto res = sp.back();
}
struct A{};
constexpr int iArr1[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
@ -71,5 +76,8 @@ int main(int, char**)
testRuntimeSpan(std::span<std::string> (&s, 1));
testRuntimeSpan(std::span<std::string, 1>(&s, 1));
std::span<int, 0> sp;
testEmptySpan(sp);
return 0;
}

9
libcxx/test/std/containers/views/span.elem/front.pass.cpp
View File

@ -38,6 +38,12 @@ void testRuntimeSpan(Span sp)
assert(std::addressof(sp.front()) == sp.data());
}
template <typename Span>
void testEmptySpan(Span sp)
{
if (!sp.empty())
[[maybe_unused]] auto res = sp.front();
}
struct A{};
constexpr int iArr1[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
@ -71,5 +77,8 @@ int main(int, char**)
testRuntimeSpan(std::span<std::string> (&s, 1));
testRuntimeSpan(std::span<std::string, 1>(&s, 1));
std::span<int, 0> sp;
testEmptySpan(sp);
return 0;
}