tests/lib/cpp/cxx: remove inline comments from DTS-generated headers

The DTS extraction process generates headers with inline comments, which
are not ignored properly by the C++ preprocessor. This is causing build
failures when using devicetree macros in C++ code.

Add a note to the C++ documentation to inform users about the issue and
how to work around it, and update the C++ test case.

Signed-off-by: Luca Burelli <[email protected]>

Author: pillo79

doc/develop/languages/cpp/index.rst

@@ -36,8 +36,17 @@ The C++ standard requires the ``main()`` function to have the return type of
 has to be selected. Zephyr ignores the return value from main, so applications
 should not return status information and should, instead, return zero.
 
+If you need to access data from the device tree in C++ files, add the following
+argument to the ``west build`` command to prevent compile-time macro expansion
+issues:
+
+.. code-block:: console
+
+   EXTRA_GEN_DEFINES_ARGS=--no-inline-comments
+
 .. note::
-    Do not use C++ for kernel, driver, or system initialization code.
+
+   Do not use C++ for kernel, driver, or system initialization code.
 
 Language Features
 *****************

tests/lib/cpp/cxx/testcase.yaml

@@ -4,6 +4,8 @@ common:
   integration_platforms:
     - mps2/an385
     - qemu_cortex_a53
+  extra_args:
+    - EXTRA_GEN_DEFINES_ARGS=--no-inline-comments
 
 tests:
   cpp.main.minimal: