[libc++] Fix ssize test that made an assumption about ptrdiff_t being 'long'

[ldionne]

Summary

On some platforms like armv7m, the size() method of containers returns
unsigned long, while ptrdiff_t is just int. Hence, std::ssize_t ends up
being long, which is not the same as ptrdiff_t. This is usually not an
issue because std::ptrdiff_t is long, so everything works out, but it
breaks on some more exotic architectures.

[mstorsjo]

returns a type that is larger than ptrdiff_t,

where ptrdiff_t is int, but size_t is still unsigned long. As a result, std::ssize returns long

Unless this is a _really_ special platform, isn't long just 32 bits in this setup? I.e. sizeof(ptrdiff_t) == sizeof(ssize_t), (and sizeof(int) == sizeof(long)), but the root cause is that int and long are two distinct different types even if their sizes happen to match?

[ldionne]

In D114563#3152482, @mstorsjo wrote:

returns a type that is larger than ptrdiff_t,

where ptrdiff_t is int, but size_t is still unsigned long. As a result, std::ssize returns long

Unless this is a _really_ special platform, isn't long just 32 bits in this setup? I.e. sizeof(ptrdiff_t) == sizeof(ssize_t), (and sizeof(int) == sizeof(long)), but the root cause is that int and long are two distinct different types even if their sizes happen to match?

You are right! I'm editing my commit message.

[ldionne]

Amend commit message.

[Mordante]

LGTM!

libcxx/test/std/iterators/iterator.container/ssize.pass.cpp

 }
 
 int main(int, char**)
 {
     std::vector<int> v; v.push_back(1);
     std::list<int>   l; l.push_back(2);
     std::array<int, 1> a; a[0] = 3;
     std::initializer_list<int> il = { 4 };
+    using SSize = std::common_type_t<std::ptrdiff_t, std::make_signed_t<std::size_t>>;
     test_container ( v );
-    ASSERT_SAME_TYPE(ptrdiff_t, decltype(std::ssize(v)));
+    ASSERT_SAME_TYPE(SSize, decltype(std::ssize(v)));
     test_container ( l );
-    ASSERT_SAME_TYPE(ptrdiff_t, decltype(std::ssize(l)));
+    ASSERT_SAME_TYPE(SSize, decltype(std::ssize(l)));
     test_container ( a );
-    ASSERT_SAME_TYPE(ptrdiff_t, decltype(std::ssize(a)));
+    ASSERT_SAME_TYPE(SSize, decltype(std::ssize(a)));
     test_container ( il );
-    ASSERT_SAME_TYPE(ptrdiff_t, decltype(std::ssize(il)));
+    ASSERT_SAME_TYPE(SSize, decltype(std::ssize(il)));
 
     test_const_container ( v );
     test_const_container ( l );
     test_const_container ( a );
     test_const_container ( il );
 
     std::string_view sv{"ABC"};
     test_container ( sv );
-    ASSERT_SAME_TYPE(ptrdiff_t, decltype(std::ssize(sv)));
+    ASSERT_SAME_TYPE(SSize, decltype(std::ssize(sv)));
     test_const_container ( sv );
 
     static constexpr int arrA [] { 1, 2, 3 };
     ASSERT_SAME_TYPE(ptrdiff_t, decltype(std::ssize(arrA)));
     static_assert( std::is_signed_v<decltype(std::ssize(arrA))>, "");
     test_const_array ( arrA );
 
 //  From P1227R2: