Thanks @Chaosus . Is this only for dealing with trailing commas (to show text editor error instead of crash)?

for (int i = 0, j = 0; i < 9 && j < 9; i++, j++)
Is comma on 3rd (continuation) part still valid?

for (int i = 0, j = 0; (i = f(i)), i < 9; i++, j++)
Is comma on 2nd (condition) part now made invalid or does it have comma operator semantics (fixing to allow non-bool before the comma)? Should I still open an issue for this one? Done at #95451

Is comma on 3rd (continuation) part still valid?

Yes

for (int i = 0, j = 0; (i = f(i)), i < 9; i++, j++)

This is not a valid because only boolean expressions are allowed on middle place. That is allowed expression in GLSL, though.

This is not a valid because only boolean expressions are allowed on middle place. That is allowed expression in GLSL, though.

Exactly. It's valid GLSL because only the last comma part needs to be a boolean (if you allow commas here like in GLSL).
If all parts are required to be a boolean, it makes it seem as if the comma means something else, like e.g. an AND or OR operator (which do get passed a boolean). Comma operator doesn't need compatible types between its parts.

Thanks!

Cherry-picked for 4.3.1.