[libc++] Use unsigned char in basic_string::__short again

[philnik]

Summary

D125496 changed the string layout on windows. Change __is_long_ and __size_ back to using unsigned char to fix the issue.

[Mordante]

LGTM modulo one comment.

libcxx/include/string

697

As I already commented on D125496 I think we should add static_asserts validating the size. That should catch these errors in our CI.

(same for the one below.)

[jloser]

libcxx/include/string

697

I agree re binding this behavior with a test to avoid surprises again in the future.

[philnik]

• Add static_assert to verify the size of __short

[philnik]

• Add message to static_assert

[philnik]

It seems like AIX is currently broken. I'll land it for now. If this breaks AIX (which it shouldn't) we'll know and I can try to fix it then.

[ldionne]

We need to add a test (in libcxx/test/libcxx/) to ensure that we can't regress this in the future.

libcxx/include/string

#ifdef _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
     enum {__min_cap = (sizeof(__long) - 1)/sizeof(value_type) > 2 ?
                       (sizeof(__long) - 1)/sizeof(value_type) : 2};
 
     struct __short
     {
         value_type __data_[__min_cap];
         unsigned char __padding_[sizeof(value_type) - 1];
-        size_type __size_ : 7;
-        size_type __is_long_ : 1;
+        unsigned char __size_ : 7;
+        unsigned char __is_long_ : 1;
     };
 
 // The __endian_factor is required because the field we use to store the size
 // has one fewer bit than it would if it were not a bitfield.
 //
 // If the LSB is used to store the short-flag in the short string representation,
 // we have to multiply the size by two when it is stored and divide it by two when
 // it is loaded to make sure that we always store an even number. In the long string
 // representation, we can ignore this because we can assume that we always allocate
struct __long
         pointer   __data_;
     };
 
     enum {__min_cap = (sizeof(__long) - 1)/sizeof(value_type) > 2 ?
                       (sizeof(__long) - 1)/sizeof(value_type) : 2};
 
     struct __short
     {
-        size_type __is_long_ : 1;
-        size_type __size_ : 7;
+        unsigned char __is_long_ : 1;
+        unsigned char __size_ : 7;
         char __padding_[sizeof(value_type) - 1];
         value_type __data_[__min_cap];
     };
 
 #endif // _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 
+    static_assert(sizeof(__short) == (sizeof(value_type) * (__min_cap + 1)), "__short has an unexpected size.");
+
     union __ulx{__long __lx; __short __lxx;};
 
     enum {__n_words = sizeof(__ulx) / sizeof(size_type)};
 
     struct __raw
     {
         size_type __words[__n_words];
     };