pw_log: Remove uses of PW_COMMA_ARGS

- Remove uses of PW_COMMA_ARGS on macro-macro boundaries. PW_COMMA_ARGS
  doesn't expand as expected when invoking one macro from another.
- Expand comment for PW_COMMA_ARGS to indicate that it should not be
  used when invoking a macro from another macro. It should only be used
  when the macro expands to C code (e.g. a function call).

Bug: 117
Change-Id: I201197f0b4727304cc2dedad40ed2f5b6d02b27e

1 files changed, 6 insertions, 1 deletions

diff --git a/pw_preprocessor/public/pw_preprocessor/macro_arg_count.h b/pw_preprocessor/public/pw_preprocessor/macro_arg_count.h
index 852557702..4d704deac 100644
--- a/pw_preprocessor/public/pw_preprocessor/macro_arg_count.h
+++ b/pw_preprocessor/public/pw_preprocessor/macro_arg_count.h
@@ -103,13 +103,18 @@
 // Expands to a comma followed by __VA_ARGS__, if __VA_ARGS__ is non-empty.
 // Otherwise, expands to nothing. This is useful when calling a function with
 // __VA_ARGS__, since it removes the extra comma when no arguments are
-// provided.
+// provided. It must NOT be used when invoking a macro from another macro.
 //
 // This is a more flexible, standard-compliant version of ##__VA_ARGS__. Unlike
 // ##__VA_ARGS__, this can be used to eliminate an unwanted comma when
 // __VA_ARGS__ expands to an empty argument because an outer macro was called
 // with __VA_ARGS__ instead of ##__VA_ARGS__.
 //
+// PW_COMMA_ARGS must NOT be used to conditionally include a comma when invoking
+// a macro from another macro. PW_COMMA_ARGS only functions correctly when the
+// macro expands to C or C++ code! When invoking one macro from another, simply
+// pass __VA_ARGS__.
+//
 // This can be used to call variadic functions or provide variadic template
 // parameters from a macro. For example:
 //