doxygen "warning: the name could not be resolved" namespace

TL;DR

Make sure the namespace's code is in Doxygen's INPUT and that the reference uses the fully qualified name. Doxygen cannot resolve what it never parsed.

Error

warning: the name 'mylib::detail' could not be resolved

Steps

1. Check the Doxyfile INPUT paths: the file defining the namespace must be included, and EXCLUDE patterns must not filter it out. Expected: the defining file is inside INPUT and outside EXCLUDE.
2. Verify the namespace is actually documented or at least declared in the parsed code; undocumented anonymous details are invisible to the resolver. Expected: the symbol exists in the parsed sources.
3. Use the fully qualified name in the referencing comment (e.g. mylib::detail::Helper), since short names resolve only within scope. Expected: references use complete paths.
4. Re-run doxygen Doxyfile and confirm the warning is gone. Expected: zero unresolved-name warnings.

When to use

• Doxygen warns a name or namespace could not be resolved
• References use short names across namespace boundaries
• INPUT was narrowed and symbols dropped out of the parse

When not to use

• The warning is about an undocumented parameter (different warning)
• Graphviz dot fails to run (an installation problem)
• The symbol is intentionally internal (document the exclusion instead)

Tool compatibility

Doxygen 1.9 through 1.12. Name resolution rules are stable across these versions.

Variant phrasings

could not be resolved for a class in another namespace

Same fix: fully qualify the name and confirm the class's file is in INPUT.

warning appears after moving headers

Update INPUT to the new header locations; stale paths silently drop symbols.

Why it happens

Doxygen resolves references against the symbols it parsed, not against your compiler's view. Anything outside INPUT, excluded, or referenced by a short name from the wrong scope fails resolution.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_PQHUQRKuVHMk1AFFnPOCjg