[libcxx] [test] Correct asserted type in subspan test; subspan with count should never produce dynamic_extent

[BillyONeal]

Summary

I'm not sure if this was a change in the standard behavior or if it was a typo in libcxx from the beginning. http://eel.is/c++draft/span.sub#10 says that if a count is supplied, the returned span is always of that count, never of dynamic_extent, so these tests were asserting incorrectly.

I'm not sure what to do about this as fixing the test to assert correctly (and thus make msvc++ pass) is going to make libcxx fail.

[ldionne]

I think this was addressed by https://reviews.llvm.org/D71997.

[BillyONeal]

@ldionne I think you're correct, though the test there is more complex than it needs to be (because the ==Count cases are the only ever encountered here).

[ldionne]

In D73138#1881198, @BillyONeal wrote:

@ldionne I think you're correct, though the test there is more complex than it needs to be (because the ==Count cases are the only ever encountered here).

Hmm, yes, you're right! I'll take your patch on top. Thanks!

[BillyONeal]

Thanks!

[miscco]

Ah sorry about that, I overlooked that we had special cased tests without count.

Thanks Billy

libcxx/test/std/containers/views/span.sub/subspan.pass.cpp

constexpr bool testConstexprSpan(Span sp)
     LIBCPP_ASSERT((noexcept(sp.template subspan<Offset, Count>())));
     LIBCPP_ASSERT((noexcept(sp.subspan(Offset, Count))));
     auto s1 = sp.template subspan<Offset, Count>();
     auto s2 = sp.subspan(Offset, Count);
     using S1 = decltype(s1);
     using S2 = decltype(s2);
     ASSERT_SAME_TYPE(typename Span::value_type, typename S1::value_type);
     ASSERT_SAME_TYPE(typename Span::value_type, typename S2::value_type);
-    if constexpr (Count != std::dynamic_extent) {
-        static_assert(S1::extent == Count);
-    } else if constexpr (Span::extent != std::dynamic_extent) {
-        static_assert(S1::extent == Span::extent - Offset);
-    } else {
-        static_assert(S1::extent == std::dynamic_extent);
-    }
     static_assert(S2::extent == std::dynamic_extent, "");
     return
         s1.data() == s2.data()
      && s1.size() == s2.size()
      && std::equal(s1.begin(), s1.end(), sp.begin() + Offset);
 }
 
 template <typename Span, size_t Offset>
void testRuntimeSpan(Span sp)
     LIBCPP_ASSERT((noexcept(sp.template subspan<Offset, Count>())));
     LIBCPP_ASSERT((noexcept(sp.subspan(Offset, Count))));
     auto s1 = sp.template subspan<Offset, Count>();
     auto s2 = sp.subspan(Offset, Count);
     using S1 = decltype(s1);
     using S2 = decltype(s2);
     ASSERT_SAME_TYPE(typename Span::value_type, typename S1::value_type);
     ASSERT_SAME_TYPE(typename Span::value_type, typename S2::value_type);
-    if constexpr (Count != std::dynamic_extent) {
-        static_assert(S1::extent == Count);
-    } else if constexpr (Span::extent != std::dynamic_extent) {
-        static_assert(S1::extent == Span::extent - Offset);
-    } else {
-        static_assert(S1::extent == std::dynamic_extent);
-    }
     static_assert(S2::extent == std::dynamic_extent, "");
     assert(s1.data() == s2.data());
     assert(s1.size() == s2.size());
     assert(std::equal(s1.begin(), s1.end(), sp.begin() + Offset));
 }
 
 
 template <typename Span, size_t Offset>