drivers: adc: fix warning when using ADC_DT_SPEC_GET macro on C++

[ArchieAtkinson]

Modifies the ADC_CHANNEL_CFG_DT macro to always initialise the differential property to prevent the -Wmissing-field-initializers warning when using ADC_DT_SPEC_GET macros in C++ when not using a differential ADC.

[github-actions]

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[marc-hb]

A similar, recent change broke one of our toolchains:

• Revert "init: add missing initialization of dev pointer in SYS_INIT (alternative to PR68125) #68118

Please hold this back until the corresponding policy has been discussed and approved and this flag added in CI.

• Error compiling C++ with flag (-Wmissing-field-initializers) #67396

This policy should be either enforced everywhere or nowhere.

See also:

• init.h: restore designated initializers in SYS_INIT_NAMED() #69411

[marc-hb]

Enforcing this policy could be as simple as making sure CI compiles every .h file with g++ -std=c++20 at least once.

https://github.com/zephyrproject-rtos/zephyr/pull/69486/files explains why.

This would of course catch ALL OTHER C++ compatibility issues, not just this initializer warning.

[ArchieAtkinson]

Enforcing this policy could be as simple as making sure CI compiles every .h file with g++ -std=c++20 at least once.

https://github.com/zephyrproject-rtos/zephyr/pull/69486/files explains why.

This would of course catch ALL OTHER C++ compatibility issues, not just this initializer warning.

As someone who uses Zephyr with C++20 in production code, I would love to see these compatibility issues solved and be part of the effort to do so. However, I think there would have to be an appetite for it within the project as macros like CONTAINER_OF() that are widely used trigger C++ warnings.

[marc-hb]

Enforcing this policy could be as simple as making sure CI compiles every .h file with g++ -std=c++20 at least once.

This could be as simple as compiling a file with a lot of #include in tests/lib/cpp. You should give it a try.

EDIT: except... macros. Unused C/C++ macros are parsed but obviously not "compiled" unless you use them :-(

[marc-hb]

This could be as simple as compiling a file with a lot of #include in tests/lib/cpp. You should give it a try.

It's a bit more complicated because unused macros are parsed but not compiled.

It can be done though, see how I'm adding C++ compilation of device.h in #70403 with just one additional line in tests/lib/cpp/cxx/src/main.cpp

See if you can actually compile ADC_CHANNEL_CFG_DT() in C++ using some tests/ change too.

EDIT: I thoroughly documented this type of compatibility issue between C and C++ in:

• doc: languages: add new section "headers and C/C++ compatibility" #74047

[github-actions]

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