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C11, Section 5.1.1.2, para. 4 [ISO/IEC 9899:2011], says,
If a character sequence that matches the syntax of a universal character name is produced by token concatenation (6.10.3.3), the behavior is undefined.
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CERT C++ Secure Coding Standard: PRE30-CPP. Do not create a universal character name through concatenation
ISO/IEC 9899:2011 Section Section 5.1.1.2, "Translation phases," Section 6.4.3, "Universal character names," and Section 6.10.3.3, "The ## operator"
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