Correct behavior for string literal concatenation (C++11 phase 6 of translation)

Question

I'm pretty sure that Visual C++ 2015 has a bug here, but I don't feel 100% sure.

Code:

// Encoding: UTF-8 with BOM (required by Visual C++).
#include <stdlib.h>

auto main()
    -> int
{
    auto const s = L""
        "𐐷 is not in the Unicode BMP!";
    return s[0] > 256? EXIT_SUCCESS : EXIT_FAILURE;
}

Result with g++:

[H:\scratchpad\simple_text_io]
> g++ --version | find "++"
g++ (i686-win32-dwarf-rev1, Built by MinGW-W64 project) 6.2.0

[H:\scratchpad\simple_text_io]
> g++ compiler_bug_demo.cpp

[H:\scratchpad\simple_text_io]
> run a
Process exit code = 0.

[H:\scratchpad\simple_text_io]
> _

Result with Visual C++:

[H:\scratchpad\simple_text_io]
> cl /nologo- 2>&1 | find "++"
Microsoft (R) C/C++ Optimizing Compiler Version 19.00.23026 for x86

[H:\scratchpad\simple_text_io]
> cl compiler_bug_demo.cpp /Feb
compiler_bug_demo.cpp
compiler_bug_demo.cpp(8): warning C4566: character represented by universal-character-name '\U00010437' cannot be represented in the current code page (1252)

[H:\scratchpad\simple_text_io]
> run b
Process exit code = 1.

[H:\scratchpad\simple_text_io]
> _

Is there any UB involved, and if not, which compiler behaves correctly?

Addendum:

The behavior is unchanged for both compilers if use lowercase greek PI, “π”, which is in the BMP, so that doesn't seem to matter.

Answer 1

From [lex.string]:

13. In translation phase 6, adjacent string literals are concatenated. If both string literals have the same encoding-prefix, the resulting concatenated string literal has that encoding-prefix. If one string literal has no encoding-prefix, it is treated as a string literal of the same encoding-prefix as the other operand. If a UTF-8 string literal token is adjacent to a wide string literal token, the program is ill-formed. Any other concatenations are conditionally-supported with implementation-defined behavior. [ Note: This concatenation is an interpretation, not a conversion. Because the interpretation happens in translation phase 6 (after each character from a literal has been translated into a value from the appropriate character set), a string literal’s initial rawness has no effect on the interpretation or well-formedness of the concatenation. —end note ] Table 8 has some examples of valid concatenations.

So there is no UB here, however phase 5 of translation might have already changed values of some characters:

5. Each source character set member in a character literal or a string literal, as well as each escape sequence and universal-character-name in a character literal or a non-raw string literal, is converted to the corresponding member of the execution character set if there is no corresponding member, it is converted to an implementation-defined member other than the null (wide) character.