The character c is actually not a decoded dchar, but the first byte from the sequence. Subsequently the test for Unicode letters fails here: https://github.com/dlang/dmd/blob/77260f78e45675bb1418feb9836ed0c6252fd1b9/src/dmd/lexer.d#L1504 So this would fail: q"übel übel"
@dkorpel created dlang/dmd pull request #14625 "Fix 19623 - HERE string identifier cannot start with Unicode letter" fixing this issue: - Fix 19623 - HERE string identifier cannot start with Unicode letter https://github.com/dlang/dmd/pull/14625
dlang/dmd pull request #14625 "Fix 19623 - HERE string identifier cannot start with Unicode letter" was merged into master: - c0403e605bee1cfcba4ba31c2f06f2b77fc769d9 by Dennis Korpel: Fix 19623 - HERE string identifier cannot start with Unicode letter https://github.com/dlang/dmd/pull/14625