`??` followed by `'`, `(`, `)` and others in a C++ string is a trigraph

PitfallVerified 27 Sep 2026Holds anywhere

Pitfall. The symptom, what causes it, and the fix that was run and seen to work.

Symptom#

An error message such as "arithmetic or '??' (duql stage 12a2)" draws a clang warning:

Trigraph ignored [-Wtrigraphs]

Cause#

??', ??(, ??), ??=, ??/, ??!, ??<, ??> and ??- are trigraphs. C++17 removed them, so clang ignores them but warns; an older standard mode or another compiler may replace them.

Fix#

Do not write ?? directly before one of those characters in a literal. Reword ("a coalesce"), or split the literal: "'?" "?'". "??" followed by " or a letter is not a trigraph.

Evidence#

dftracer-utils duql lowering (2026-09-27): clangd reported the warning on src/dftracer/utils/duql/lower.cpp; after the rewording the full tests build had no warnings.