According to MISRA 2008, concatenation of wide and narrow string literals leads to undefined behavior. This was once considered implicitly undefined behavior until C90 [ISO/IEC 9899:1990]. However, the C Standard C99 defined this behavior [ISO/IEC 9899:20111999] does define this behavior, and it is C11 further explained explains in section 6.4.5, paragraph 5 [ISO/IEC 9899:2011]:
In translation phase 6, the multibyte character sequences specified by any sequence of adjacent character and identically-prefixed string literal tokens are concatenated into a single multibyte character sequence. If any of the tokens has an encoding prefix, the resulting multibyte character sequence is treated as having the same prefix; otherwise, it is treated as a character string literal. Whether differently-prefixed wide string literal tokens can be concatenated and, if so, the treatment of the resulting multibyte character sequence are implementation-defined.
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