Suppress GCC 16 incomplete type warning

[TheNonConformist]

GCC 16 emits -Wsfinae-incomplete when the incomplete type used by
incomplete_type_test is later defined.

The pattern is intentional because the test verifies formatting a type while
it is still incomplete. Suppress the GCC 16 diagnostic narrowly around the
definition so the test retains its coverage without producing a warning.

Tested with GCC 16.2.1:

• Full build succeeded
• 21/21 tests passed
• git diff --check passed

[vitaut]

Could you post the full warning with context and the repro command (or godbolt link) for future reference?

[TheNonConformist]

Could you post the full warning with context and the repro command (or godbolt link) for future reference?

Sure. I reproduced it with GCC 16.2.1 using:

cmake -S . -B build -DFMT_TEST=ON -DFMT_DOC=OFF -DFMT_INSTALL=OFF
cmake --build build --target format-test --clean-first --verbose

The complete diagnostic is: