Hi Rasmus, On Wed, Aug 05, 2020 at 01:38:58PM +0200, Rasmus Villemoes wrote: > I'm guessing gcc has some internal very early simplification that > replaces single-expression statement-exprs with just that expression, > and the warn-unused-result triggers later. But as soon as the > statement-expr becomes a little non-trivial (e.g. above), my guess is > that the whole thing gets assigned to some internal "variable" > representing the result, and that assignment then counts as a use of the > return value from must_check_overflow() - cc'ing Segher, as he usually > knows these details.
A statement expression is not a statement (it's an expression), which turns half of the world upside down. This GCC extension often has weird (or at least non-intuitive) side effects, together with other extensions (like attributes), etc. This may be a convoluted way of saying "I don't know, look at c/c-decl.c (and maybe c/c-parser.c) to see if you can find out" ;-) > Anyway, we don't need to apply it to the last expression inside ({}), we > can just pass the whole ({}) to must_check_overflow() as in <snip> Yes, much nicer :-) Crisis averted, etc. Segher