Would you say that the C macro ... performs implicit conversion

Yes. The code is in the macro, not outside the macro. If you're looking at it from the perspective of the code using the macro, the conversion is implicit. If you're looking at it from the perspective of the "+", the conversion is explicit.

Would you not characterize this code as clunky?

Yes. It's clunky because there's no need for explicit conversion — it's being done implicitly. Let the concatenation operator coerce the operand.

You've said coercion occurs and you've shown that explicit conversion is unnecessary. I have no idea what you are arguing.

• Do you think a scalar is being converted to a number?
• Do you think the addition operator does this conversion or its caller?