The issue is with the way tokenization works. The gcc documentation has a page about tokenization and concatenation from which I glean the following:
The preprocessor separates the source code into tokens which are roughly equivalent to the C compiler tokens (e.g. literals, identifiers or operators). These tokens can be combined with the ## operator. The result of the concatenation must in turn be a token (for example, an identifier-shaped thing, a number, an operator), which is not the case here, which is why the compiler complains.
The key is that the character sequence [0]is separated into the three-token sequence[, 0, ]. The concatenation operator uses only [ as its right hand side operator; the result x[ is not a token.
As a counter-example, the following works:
#define MAKE_ASSIGN_OP(x) x##=int main() { int x=1; return x MAKE_ASSIGN_OP(+) 1; }Both + and = as well as their concatenation += are valid tokens.
As an example