[Moved to DR at the October, 2012 meeting.]

Proposed resolution (February, 2012):

Change 7.6.7 [expr.shift] paragraph 2 as follows:

...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; otherwise, the behavior is undefined.

The current wording of 7.6.7 [expr.shift] paragraph 2 makes it undefined behavior to create the most-negative integer of a given type by left-shifting a (signed) 1 into the sign bit, even though this is not uncommonly done and works correctly on the majority of (twos-complement) architectures:

...if E1 has a signed type and non-negative value, and E1 ⨯ 2^E2 is representable in the result type, then that is the resulting value; otherwise, the behavior is undefined.

As a result, this technique cannot be used in a constant expression, which will break a significant amount of code.