Comma-first is absolutely conventional in many parts of literally millions of C++ files, particularly for member initializers in constructor definitions, but of

Comma-first is absolutely conventional in many parts of literally millions of C++ files, particularly for member initializers in constructor definitions, but often, also, argument lists. It makes for cleaner edits and diffs. If you are designing a syntax that does not rely on line breaks like Python, reach for "introducers" instead of "delimiters". Commas make awkward introducers even though people can and do get used to it.