(From submission #511.)

Subclause 5.13.5 [lex.string] paragraph 7 does not properly handle the case where both encoding-prefixes are absent:

The common encoding-prefix for a sequence of adjacent string-literals is determined pairwise as follows: If two string-literals have the same encoding-prefix , the common encoding-prefix is that encoding-prefix . If one string-literal has no encoding-prefix , the common encoding-prefix is that of the other string-literal. Any other combinations are ill-formed.

Possible resolution:

The common encoding-prefix for a sequence of adjacent string-literals is determined pairwise as follows: If two string-literals have the same encoding-prefix, the common encoding-prefix is that encoding-prefix. If one string-literal has no encoding-prefix, the common encoding-prefix is that of the other string-literal or is absent if the other string-literal has no encoding-prefix. Otherwise, both string-literals shall have the same encoding-prefix, and the common encoding-prefix is that encoding-prefix. Any other combinations are ill-formed.