[libcxx][test] Fix span tests.

span.cons/container.pass.cpp
N4842 22.7.3.2 [span.cons]/13 constrains span's range constructor
for ranges::contiguous_range (among other criteria).

24.4.5 [range.refinements]/2 says that contiguous_range requires data(),
and (via contiguous_range, random_access_range, bidirectional_range,
forward_range, input_range, range) it also requires begin() and end()
(see 24.4.2 [range.range]/1).

Therefore, IsAContainer needs to provide begin() and end().

(Detected by MSVC's concept-constrained implementation.)

span.cons/stdarray.pass.cpp
This test uses std::array, so it must include <array>.
<span> isn't guaranteed to drag in <array>.

(Detected by MSVC's implementation which uses a forward declaration to
avoid dragging in <array>, for increased compiler throughput.)

span.objectrep/as_bytes.pass.cpp
span.objectrep/as_writable_bytes.pass.cpp
Testing `sp.extent == std::dynamic_extent` triggers MSVC warning
C4127 "conditional expression is constant". Using `if constexpr` is a
simple way to avoid this without disrupting anyone else (as span
requires C++20 mode).

span.tuple/get.pass.cpp
22.7.3.2 [span.cons]/4.3: "Preconditions: If extent is not equal to
dynamic_extent, then count is equal to extent."

These lines were triggering undefined behavior (detected by assertions
in MSVC's implementation).

I changed the count arguments in the first two chunks, followed by
changing the span extents, in order to preserve the test's coverage
and follow the existing pattern.

span.cons/span.pass.cpp
22.7.3.2 [span.cons]/18.1 constrains span's converting constructor with
"Extent == dynamic_extent || Extent == OtherExtent is true".

This means that converting from dynamic extent to static extent is
not allowed. (Other constructors tested elsewhere, like
span(It first, size_type count), can be used to write such code.)

As this is the test for the converting constructor, I have:

* Removed the "dynamic -> static" case from checkCV(), which is
comprehensive.

* Changed the initialization of std::span<T, 0> s1{}; in
testConstexprSpan() and testRuntimeSpan(), because s1 is used below.

* Removed ASSERT_NOEXCEPT(std::span<T, 0>{s0}); from those functions,
as they are otherwise comprehensive.

* Deleted testConversionSpan() entirely. Note that this could never
compile (it had a bool return type, but forgot to say `return`). And it
couldn't have provided useful coverage, as the /18.2 constraint
"OtherElementType(*)[] is convertible to ElementType(*)[]"
permits only cv-qualifications, which are already tested by checkCV().

1 files changed, 1 insertions, 1 deletions

diff --git a/libcxx/test/std/containers/views/span.objectrep/as_bytes.pass.cpp b/libcxx/test/std/containers/views/span.objectrep/as_bytes.pass.cpp
index ff1038e0b11..6d757f35556 100644
--- a/libcxx/test/std/containers/views/span.objectrep/as_bytes.pass.cpp
+++ b/libcxx/test/std/containers/views/span.objectrep/as_bytes.pass.cpp
@@ -33,7 +33,7 @@ void testRuntimeSpan(Span sp)
     using SB = decltype(spBytes);
     ASSERT_SAME_TYPE(const std::byte, typename SB::element_type);
 
-    if (sp.extent == std::dynamic_extent)
+    if constexpr (sp.extent == std::dynamic_extent)
         assert(spBytes.extent == std::dynamic_extent);
     else
         assert(spBytes.extent == sizeof(typename Span::element_type) * sp.extent);