On Mon, Oct 5, 2026 at 10:40 AM Jonathan Wakely <[email protected]> wrote:
> Typo in "uniquness" in the subject line.
>
> On Fri, 24 Jul 2026 at 15:09 +0200, Tomasz Kamiński wrote:
> >As indicated by issue LWG4606, the previous wording for checking
> uniqueness
> >of the mapping was incorrect, and rejected some valid combination of
> strides.
> >
> >Firstly, we observe that if new mapping N is produced by adding dimension
> >(with at least two elements) to some unique mapping M, for N to remain
> unique,
> >the stride on the newly added dimension needs to be greater than to
> maximum
> >index produced by M, i.e:
> > sum_[0,M::rank()) M.stride(1) * (M.extents().extent() - 1)
> >As M.required_span_size() is 1 + above sum, equivalently stride needs to
> be
> >greater than or equal to it.
> >
> >This patch, uses above to find permutation of dimension index (__perm),
> >corresponding to order in which dimension are appended (for extent greater
> >than one, this is equivalent to lowest to highest value of strides).
> >Starting from zero-dimensional mapping (__cur_size == 1), in each
> iteration
> >we compare the next __stride against required_span_size (__cur_span), and
> >if check passes, we update it's value. The above also guarantees that are
> >__stride values are greater than zero.
> >
> >As dimension of extent one (with one element), can be used only with index
> >value of 0, they stride do not contribute to index value, and can be
> >selected arbitrary (including having value zero). We handle above by
> moving
> >indices corresponding such dimensions to front of __perm. To support that
>
> "corresponding to"
>
> >__popmin is modified to accept mapping from index to value.
> >
> >As mentioned above, stride value zero are acceptable also for dimension
> >with extent equal to one (in addition to empty mappings). We remove
> >unconditional check rejecting zero strides, and replace it with invocation
> >of __stride_nonzero. This is done separately for mappings of rank one,
> two,
> >three and larger. In the last case, a separate loop is only required in
> >non-debug mode.
> >
> >As for the two dimension mappings, the above sum reduces to stride *
> >(extent - 1), we continue validating them in non-debug mode. We handle
> >degenerate mappings (at least one extent is one) first, reusing
> >__stride_nonzero to check other dimension.
> >
> >libstdc++-v3/ChangeLog:
> >
> > * include/std/mdspan (layout_stride::mapping::_M_check_unique):
> > Change the algorithm to correctly recognize examples from LWG4606
> > as unique.
> > (layout_stride::mapping(const extents_type&, span<....>)):
> > Assert on _M_check_unique only for non-zero ranks.
> > * testsuite/23_containers/mdspan/layouts/stride_neg.cc:
> > Expand tests cases and moved some examples to...
> > * testsuite/23_containers/mdspan/layouts/stride_debug_neg.cc:
> > Separate test for issues detected only in debug mode.
> >---
> >I have separate stride_debug_neg as DejaGNU was giving me strange failure
> >on dg-bogus line, when there was error on that line on mdspan header. It
> >is also easier to read the output that way.
> >
> >Given the little induction based "proof" for the condition above, I am
> >pretty certain that I got it correctly.
> >
> >Testing on x86_64-linux. *mdspan* test passed in 32bits, debug and
> >all standard modes. OK for trunk, either now or after issue is approved?
>
> OK for trunk with the typos above and below fixed, thanks.
>
I would backport it to 16 together with r17-2356-g1a4d8dc0d2dc04, after
letting it sit for some time. OK for 16 for both?
> > libstdc++-v3/include/std/mdspan | 63 ++++++++++------
> > .../mdspan/layouts/stride_debug_neg.cc | 42 +++++++++++
> > .../mdspan/layouts/stride_neg.cc | 71 ++++++++-----------
> > 3 files changed, 114 insertions(+), 62 deletions(-)
> > create mode 100644
> libstdc++-v3/testsuite/23_containers/mdspan/layouts/stride_debug_neg.cc
> >
> >diff --git a/libstdc++-v3/include/std/mdspan
> b/libstdc++-v3/include/std/mdspan
> >index 025ce4d75ab..08edcb905f4 100644
> >--- a/libstdc++-v3/include/std/mdspan
> >+++ b/libstdc++-v3/include/std/mdspan
> >@@ -1894,7 +1894,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > _M_strides[__i] =
> >
> __mdspan::__index_type_cast<index_type>(as_const(__strides[__i]));
> >
> >- __glibcxx_assert(_M_check_unique());
> >+ if constexpr (extents_type::rank() > 0)
> >+ __glibcxx_assert(_M_check_unique());
> > }
> >
> > template<typename _OIndexType>
> >@@ -2036,19 +2037,29 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > _M_check_unique() const
> > {
> > constexpr size_t __rank = extents_type::rank();
> >- if constexpr (__rank == 0)
> >- return true;
> > // _GLIBCXX_RESOLVE_LIB_DEFECTS
> > // 4603. `layout_stride` should accept zero strides for empty
> `extents`
> >- else if (__mdspan::__empty(_M_extents))
> >+ // 4606. Bullet (4.3) preconditions are too strict to implement
> submdspan
> >+
> >+ if (__mdspan::__empty(_M_extents))
> > return true;
> >- else if constexpr (__rank == 1)
> >- return _M_strides[0] > 0;
> >- else if (__mdspan::__contains_zero<const index_type,
> __rank>(_M_strides))
> >- return false;
> >+
> >+ auto __stride_nonzero = [&](size_t __idx)
> >+ { return (_M_strides[__idx] > 0) || (_M_extents.extent(__idx) ==
> 1); };
> >+
> >+ if constexpr (__rank == 1)
> >+ return __stride_nonzero(0);
> > else if constexpr (__rank == 2)
> >- return (_M_strides[1] >= _M_strides[0] * _M_extents.extent(0))
> >- || (_M_strides[0] >= _M_strides[1] * _M_extents.extent(1));
> >+ {
> >+ if (_M_extents.extent(0) == 1)
> >+ return __stride_nonzero(1);
> >+ if (_M_extents.extent(1) == 1)
> >+ return __stride_nonzero(0);
> >+ if (!_M_strides[0] || !_M_strides[1])
> >+ return false;
> >+ return (_M_strides[1] > _M_strides[0] * (_M_extents.extent(0)
> - 1))
> >+ || (_M_strides[0] > _M_strides[1] * (_M_extents.extent(1)
> - 1));
> >+ }
> > else
> > {
> > // This is necessary, but not sufficient condition for
> uniqueness.
> >@@ -2064,15 +2075,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > for (size_t __i = 0; __i < __rank; ++__i)
> > __perm[__i] = __i;
> >
> >- auto __popmin = [&](size_t __from)
> >+ auto __getstride = [&](size_t __idx) { return
> _M_strides[__idx]; };
> >+ auto __getextent = [&](size_t __idx) { return
> _M_extents.extent(__idx); };
> >+ auto __popmin = [&](size_t __from, auto __getter)
> > {
> > size_t __min = __perm[__rank - 1];
> > for (size_t __i = __rank - 1; __i > __from; --__i)
> > {
> > const size_t __cur = __perm[__i - 1];
> >- const auto __res = _M_strides[__cur] <=>
> _M_strides[__min];
> >- // Equal strides are allowed only for extent one, so
> apply it first.
> >- if (__res < 0 || (__res == 0 && _M_extents.extent(__cur)
> == 1))
> >+ if (__getter(__cur) < __getter(__min))
> > {
> > __perm[__i] = __min;
> > __min = __cur;
> >@@ -2081,18 +2092,30 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > __perm[__i] = __cur;
> > }
> > __perm[__from] = __min;
> >- return __min;
> >+ return __getter(__min);
> > };
> >
> > // We use eager insertion-sort based algorithm to permute the
> __perm.
> >- size_t __prev = __popmin(0);
> >- for (size_t __i = 1; __i < __rank; ++__i)
> >+ size_t __idx = 0;
> >+
> >+ // Stride valus are not revelant for dimension with single
> element
>
> "values" and "relevant"
>
> >+ while (__popmin(__idx, __getextent) == 1)
> >+ if (++__idx == __rank)
> >+ return true;
> >+
> >+ size_t __prev_span = 1;
> >+ for (; __idx < __rank; ++__idx)
> > {
> >- const size_t __next = __popmin(__i);
> >- if (_M_strides[__next] < _M_strides[__prev] *
> _M_extents.extent(__prev))
> >+ const size_t __stride = __popmin(__idx, __getstride);
> >+ if (__stride < __prev_span)
> > return false;
> >- __prev = __next;
> >+ __prev_span += __stride * (__getextent(__perm[__idx]) - 1);
> > }
> >+#else
> >+ // Otherwise check obvious cases where stride is zero.
> >+ for (size_t __idx = 0; __idx < __rank; ++__idx)
> >+ if (!__stride_nonzero(__idx))
> >+ return false;
> > #endif
> > return true;
> > }
> >diff --git
> a/libstdc++-v3/testsuite/23_containers/mdspan/layouts/stride_debug_neg.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/stride_debug_neg.cc
> >new file mode 100644
> >index 00000000000..bc5f41489d7
> >--- /dev/null
> >+++
> b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/stride_debug_neg.cc
> >@@ -0,0 +1,42 @@
> >+// { dg-do compile { target c++23 } }
> >+#include <mdspan>
> >+#include <cstdint>
> >+
> >+using std::array;
> >+template<size_t Rank>
> >+ using mapping = std::layout_stride::mapping<std::dextents<size_t,
> Rank>>;
> >+
> >+// Only some permutations are valid, invalid permutations detected only
> in debug mode.
> >+
> >+// Unique if stride 4 corresponds to extent 3
> >+constexpr auto m28 = mapping<2>(array{3, 4}, array{4, 10}); // {
> dg-bogus "expansion of" }
> >+constexpr auto m29 = mapping<2>(array{3, 4}, array{10, 4}); // {
> dg-error "expansion of" "" { target debug_mode } }
> >+constexpr auto m30 = mapping<2>(array{4, 3}, array{10, 4}); // {
> dg-bogus "expansion of" }
> >+constexpr auto m31 = mapping<2>(array{4, 3}, array{4, 10}); // {
> dg-error "expansion of" "" { target debug_mode } }
> >+
> >+// Unique if stride 11 corresponds to extent 3, 2 * (5-1) = 8 < 11, and
> 11 * (3-1) + 8 = 30 < 35
> >+constexpr auto m32 = mapping<3>(array{3, 4, 5}, array{11, 2, 35}); // {
> dg-bogus "expansion of" }
> >+constexpr auto m33 = mapping<3>(array{3, 4, 5}, array{11, 35, 2}); // {
> dg-bogus "expansion of" }
> >+constexpr auto m34 = mapping<3>(array{3, 4, 5}, array{2, 11, 35}); // {
> dg-error "expansion of" "" { target debug_mode } }
> >+constexpr auto m35 = mapping<3>(array{3, 4, 5}, array{35, 2, 11}); // {
> dg-error "expansion of" "" { target debug_mode } }
> >+constexpr auto m36 = mapping<3>(array{5, 4, 3}, array{2, 35, 11}); // {
> dg-bogus "expansion of" }
> >+constexpr auto m37 = mapping<3>(array{5, 4, 3}, array{2, 11, 35}); // {
> dg-error "expansion of" "" { target debug_mode } }
> >+constexpr auto m38 = mapping<3>(array{5, 3, 4}, array{35, 11, 2}); // {
> dg-bogus "expansion of" }
> >+constexpr auto m39 = mapping<3>(array{3, 5, 4}, array{35, 2, 11}); // {
> dg-error "expansion of" "" { target debug_mode } }
> >+
> >+// Unique for stride -> extent mapping: 3 * (5-1) = 12 < 16, 15 * (4-1)
> + 12 = 57 < 65, 65 * (3-1) + 57 = 187 < 200
> >+constexpr auto m40 = mapping<4>(array{3, 4, 5, 6}, array{65, 16, 3,
> 200}); // { dg-bogus "expansion of" }
> >+constexpr auto m41 = mapping<4>(array{3, 4, 5, 6}, array{3, 16, 65,
> 200}); // { dg-error "expansion of" "" { target debug_mode } }
> >+constexpr auto m42 = mapping<4>(array{3, 4, 5, 6}, array{200, 65, 3,
> 16}); // { dg-error "expansion of" "" { target debug_mode } }
> >+constexpr auto m43 = mapping<4>(array{5, 3, 6, 4}, array{3, 65, 200,
> 16}); // { dg-bogus "expansion of" }
> >+constexpr auto m44 = mapping<4>(array{5, 3, 6, 4}, array{3, 16, 200,
> 65}); // { dg-error "expansion of" "" { target debug_mode } }
> >+
> >+// Examples based on one from LWG 4606
> >+constexpr auto m45 = mapping<2>(array{2, 3}, array{5, 2}); // { dg-bogus
> "expansion of" }
> >+constexpr auto m46 = mapping<3>(array{2, 3, 5}, array{5, 2, 10}); // {
> dg-bogus "expansion of" }
> >+constexpr auto m47 = mapping<3>(array{2, 3, 1}, array{5, 2, 99}); // {
> dg-bogus "expansion of" }
> >+constexpr auto m48 = mapping<4>(array{2, 3, 5, 4}, array{5, 2, 10, 50});
> // { dg-bogus "expansion of" }
> >+constexpr auto m49 = mapping<4>(array{2, 1, 3, 5}, array{5, 99, 2, 10});
> // { dg-bogus "expansion of" }
> >+
> >+// { dg-prune-output "non-constant condition for static assertion" }
> >+// { dg-prune-output "__glibcxx_assert_fail()" }
> >diff --git
> a/libstdc++-v3/testsuite/23_containers/mdspan/layouts/stride_neg.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/stride_neg.cc
> >index 692caeac812..5a818042b5a 100644
> >--- a/libstdc++-v3/testsuite/23_containers/mdspan/layouts/stride_neg.cc
> >+++ b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/stride_neg.cc
> >@@ -19,12 +19,11 @@ constexpr bool
> > test_stride_negative()
> > {
> > auto exts = std::extents<std::size_t, dyn, dyn>(1, 3);
> >- auto m = std::layout_stride::mapping(exts, std::array{1, -4}); // {
> dg-error "expansion of" }
> >+ auto m = std::layout_stride::mapping(exts, std::array{1, -4}); // {
> dg-error "expansion of" }
> > (void) m;
> > return true;
> > }
> >-static_assert(test_stride_negative()); // { dg-error "expansion of" }
> >-
> >+static_assert(test_stride_negative()); // { dg-error "expansion of" }
> >
> > using std::array;
> > template<size_t Rank>
> >@@ -34,17 +33,25 @@ constexpr auto m0 = mapping<0>(array<int, 0>{},
> array<int, 0>{}); // { dg-bogus
> >
> > // Zero strides allowed for empty mappings
> > constexpr auto m00 = mapping<1>(array{0}, array{0}); // { dg-bogus
> "expansion of" }
> >-constexpr auto m01 = mapping<2>(array{0, 1}, array{3, 0}); // { dg-bogus
> "expansion of" }
> >-constexpr auto m02 = mapping<2>(array{0, 1}, array{0, 0}); // { dg-bogus
> "expansion of" }
> >-constexpr auto m03 = mapping<3>(array{0, 1, 2}, array{3, 0, 0}); // {
> dg-bogus "expansion of" }
> >-constexpr auto m04 = mapping<3>(array{0, 1, 2}, array{3, 3, 0}); // {
> dg-bogus "expansion of" }
> >-constexpr auto m05 = mapping<4>(array{0, 1, 2, 3}, array{5, 3, 0, 2});
> // { dg-bogus "expansion of" }
> >+constexpr auto m01 = mapping<2>(array{0, 2}, array{3, 0}); // { dg-bogus
> "expansion of" }
> >+constexpr auto m02 = mapping<2>(array{0, 2}, array{0, 0}); // { dg-bogus
> "expansion of" }
> >+constexpr auto m03 = mapping<3>(array{0, 2, 3}, array{3, 0, 0}); // {
> dg-bogus "expansion of" }
> >+constexpr auto m04 = mapping<3>(array{0, 2, 3}, array{3, 3, 0}); // {
> dg-bogus "expansion of" }
> >+constexpr auto m05 = mapping<4>(array{0, 2, 3, 4}, array{5, 3, 0, 2});
> // { dg-bogus "expansion of" }
> >+
> >+// On dimensions with stride equal to 1
> >+constexpr auto m0a = mapping<1>(array{1}, array{0}); // { dg-bogus
> "expansion of" }
> >+constexpr auto m0b = mapping<2>(array{2, 1}, array{3, 0}); // { dg-bogus
> "expansion of" }
> >+constexpr auto m0c = mapping<2>(array{1, 1}, array{0, 0}); // { dg-bogus
> "expansion of" }
> >+constexpr auto m0d = mapping<3>(array{2, 1, 1}, array{3, 0, 0}); // {
> dg-bogus "expansion of" }
> >+constexpr auto m0e = mapping<3>(array{2, 2, 1}, array{6, 3, 0}); // {
> dg-bogus "expansion of" }
> >+constexpr auto m0f = mapping<4>(array{2, 2, 1, 2}, array{8, 4, 0, 2});
> // { dg-bogus "expansion of" }
> >
> > // Otherwise leads to non-unique mappings.
> >-constexpr auto m06 = mapping<1>(array{1}, array{0});
> // { dg-error "expansion of" }
> >-constexpr auto m07 = mapping<2>(array{1, 2}, array{3, 0});
> // { dg-error "expansion of" }
> >-constexpr auto m08 = mapping<3>(array{1, 2, 3}, array{3, 3, 0});
> // { dg-error "expansion of" }
> >-constexpr auto m09 = mapping<4>(array{1, 2, 3, 5}, array{5, 3, 0, 2});
> // { dg-error "expansion of" }
> >+constexpr auto m06 = mapping<1>(array{2}, array{0}); // { dg-error
> "expansion of" }
> >+constexpr auto m07 = mapping<2>(array{2, 3}, array{3, 0}); // { dg-error
> "expansion of" }
> >+constexpr auto m08 = mapping<3>(array{2, 3, 4}, array{3, 3, 0}); // {
> dg-error "expansion of" }
> >+constexpr auto m09 = mapping<4>(array{2, 3, 4, 5}, array{5, 3, 0, 2});
> // { dg-error "expansion of" }
> >
> > // required_span_size is smaller size of multidimensional index space
> > constexpr auto m10 = mapping<2>(array{3, 4}, array{2, 3}); // { dg-error
> "expansion of" }
> >@@ -61,39 +68,19 @@ constexpr auto m18 = mapping<3>(array{3, 4, 5},
> array{1, 5, 12}); // { dg-error
> > constexpr auto m19 = mapping<4>(array{3, 4, 5, 6}, array{1, 2, 12,
> 100}); // { dg-error "expansion of" "" { target debug_mode } }
> > constexpr auto m20 = mapping<4>(array{3, 4, 5, 6}, array{1, 5, 12,
> 60}); // { dg-error "expansion of" "" { target debug_mode } }
> >
> >-// Equal indicies allowed if extents are 1
> >-constexpr auto m21 = mapping<2>(array{5, 1}, array{1, 1}); // { dg-bogus
> "expansion of" }
> >-constexpr auto m22 = mapping<3>(array{1, 5, 1}, array{1, 1, 1}); // {
> dg-bogus "expansion of" }
> >-constexpr auto m23 = mapping<4>(array{1, 1, 5, 1}, array{1, 1, 1, 1});
> // { dg-bogus "expansion of" }
> >+// Any stride values are allowed for extents 1
> >+constexpr auto m21 = mapping<2>(array{1, 1}, array{88, 99}); // {
> dg-bogus "expansion of" }
> >+constexpr auto m2a = mapping<2>(array{5, 1}, array{88, 99}); // {
> dg-bogus "expansion of" }
> >+constexpr auto m22 = mapping<3>(array{1, 1, 1}, array{88, 99, 00}); // {
> dg-bogus "expansion of" }
> >+constexpr auto m2b = mapping<3>(array{1, 5, 1}, array{88, 99, 77}); // {
> dg-bogus "expansion of" }
> >+constexpr auto m23 = mapping<4>(array{1, 1, 1, 1}, array{77, 99, 00,
> 88}); // { dg-bogus "expansion of" }
> >+constexpr auto m2c = mapping<4>(array{1, 1, 5, 1}, array{77, 99, 1,
> 88}); // { dg-bogus "expansion of" }
> > constexpr auto m24 = mapping<5>(array{3, 4, 1, 1, 5}, array{3, 10, 3, 3,
> 45}); // { dg-bogus "expansion of" }
> > constexpr auto m25 = mapping<5>(array{3, 4, 1, 1, 5}, array{3, 10, 3,
> 10, 45}); // { dg-bogus "expansion of" }
> >-constexpr auto m26 = mapping<5>(array{3, 4, 1, 1, 5}, array{3, 10, 10,
> 3, 45}); // { dg-bogus "expansion of" }
> >+constexpr auto m26 = mapping<5>(array{3, 4, 1, 1, 5}, array{3, 10, 10,
> 3, 45}); // { dg-bogus "expansion of" }
> > constexpr auto m27 = mapping<5>(array{3, 4, 1, 1, 5}, array{3, 10, 10,
> 10, 45}); // { dg-bogus "expansion of" }
> >-
> >-// Only some permutations are valid, invalid permutations detected only
> in debug mode.
> >-
> >-// Unique if stride 3 corresponds to extent 3
> >-constexpr auto m28 = mapping<2>(array{3, 4}, array{3, 10}); // {
> dg-bogus "expansion of" }
> >-constexpr auto m29 = mapping<2>(array{3, 4}, array{10, 3}); // {
> dg-error "expansion of" "" { target debug_mode } }
> >-constexpr auto m30 = mapping<2>(array{4, 3}, array{10, 3}); // {
> dg-bogus "expansion of" }
> >-constexpr auto m31 = mapping<2>(array{4, 3}, array{3, 10}); // {
> dg-error "expansion of" "" { target debug_mode } }
> >-
> >-// Unique if stride 11 corresponds to extent 3, as 11 * 3 < 35, and 2 *
> 5 < 11
> >-constexpr auto m32 = mapping<3>(array{3, 4, 5}, array{11, 2, 35}); // {
> dg-bogus "expansion of" }
> >-constexpr auto m33 = mapping<3>(array{3, 4, 5}, array{11, 35, 2}); // {
> dg-bogus "expansion of" }
> >-constexpr auto m34 = mapping<3>(array{3, 4, 5}, array{2, 11, 35}); // {
> dg-error "expansion of" "" { target debug_mode } }
> >-constexpr auto m35 = mapping<3>(array{3, 4, 5}, array{35, 2, 11}); // {
> dg-error "expansion of" "" { target debug_mode } }
> >-constexpr auto m36 = mapping<3>(array{5, 4, 3}, array{2, 35, 11}); // {
> dg-bogus "expansion of" }
> >-constexpr auto m37 = mapping<3>(array{5, 4, 3}, array{2, 11, 35}); // {
> dg-error "expansion of" "" { target debug_mode } }
> >-constexpr auto m38 = mapping<3>(array{5, 3, 4}, array{35, 11, 2}); // {
> dg-bogus "expansion of" }
> >-constexpr auto m39 = mapping<3>(array{3, 5, 4}, array{35, 2, 11}); // {
> dg-error "expansion of" "" { target debug_mode } }
> >-
> >-// Unique for stride -> extent mapping: 3 * 5 < 16, 15 * 4 < 65, 65 * 3
> < 200
> >-constexpr auto m40 = mapping<4>(array{3, 4, 5, 6}, array{65, 16, 3,
> 200}); // { dg-bogus "expansion of" }
> >-constexpr auto m41 = mapping<4>(array{3, 4, 5, 6}, array{3, 16, 65,
> 200}); // { dg-error "expansion of" "" { target debug_mode } }
> >-constexpr auto m42 = mapping<4>(array{3, 4, 5, 6}, array{200, 65, 3,
> 16}); // { dg-error "expansion of" "" { target debug_mode } }
> >-constexpr auto m43 = mapping<4>(array{5, 3, 6, 4}, array{3, 65, 200,
> 16}); // { dg-bogus "expansion of" }
> >-constexpr auto m44 = mapping<4>(array{5, 3, 6, 4}, array{3, 16, 200,
> 65}); // { dg-error "expansion of" "" { target debug_mode } }
> >+constexpr auto m2d = mapping<5>(array{3, 4, 1, 1, 5}, array{3, 10, 00,
> 99, 45}); // { dg-bogus "expansion of" }
> >+constexpr auto m2e = mapping<5>(array{1, 1, 1, 1, 1}, array{00, 77, 88,
> 99, 00}); // { dg-bogus "expansion of" }
> >
> > // { dg-prune-output "non-constant condition for static assertion" }
> > // { dg-prune-output "__glibcxx_assert_fail()" }
> >--
> >2.55.0
> >
> >
>
>
