[Bug c++/52119] [C++11] overflow in signed left shift isn't diagnosed

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52119

--- Comment #11 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to ajf from comment #10)
> MY apologies if I am missing something obvious, but what exactly makes
> shifting a 1 into the sign bit undefined behavior?

You're right, it isn't:

5.8/2: ... if E1 has a signed type and non-negative value, and E1 × 2^E2 is
representable in the corresponding unsigned type of the result type, then that
value, converted to the result type, is the resulting value ....