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The C Standard, 5.1.1.2, paragraph 4 [ISO/IEC 9899:20112024], says
If a character sequence that matches the syntax of a universal character name is produced by token concatenation (6.10.35.3), the behavior is undefined.
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| [ISO/IEC 10646-2003] | |
| [ISO/IEC 9899:20112024] | Subclause 5.1.1.2, "Translation Phases" |
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