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No, it doesn't for the simplest possible case:     #include     int main() { std::array arr; }; But admittedly that would have been a surprise to me as I usually compile with -Wall -Wextra -Werror. But I tried to replace `` with `` in yet another code (tiny but real-life, so not as trivial as above) with two appearances of `std::array` and there, one does yield the note. I boiled it down to the following: The typename seems to not yield the note but the constructor does notify, i.e. in     1 std::array f() {     2    return std::array{{1}};     3 } Line 1 and 2 raise an error but only Line 2 comes with a note. Best, Julian On 05/08/2022 17:55, Jonathan Wakely wrote: > On Fri, 5 Aug 2022 at 17:39, Julian Lenz via Gcc wrote: >> Hi everybody, >> >> TL;DR: >> >> What is the reason that `error: '' has incomplete type` does >> not give a note about where the forward declaration happened for >> standard library classes? > Probably because the declaration happens in a system header. > > Does -Wsystem-headers change it?