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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, C99 defined this behavior [ISO/IEC 9899:1999], and C11 further explains in section subclause 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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If the concatenated string needs to be a wide string literal, each element in the concatenation must be a wide string literal, as in this compliant solution.:

wchar_t *msg = L"This message is very long, so I want to divide it "
               L"into two parts.";

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If wide string literals are unnecessary, it is better to use narrow string literals, as in this compliant solution.:

char *msg = "This message is very long, so I want to divide it "
            "into two parts.";

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The concatenation of wide and narrow string literals could lead to undefined behavior.

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