[Bug c++/77875] C++ core issue 1288
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77875 Andrew Pinski changed: What|Removed |Added CC||vanyacpp at gmail dot com --- Comment #6 from Andrew Pinski --- *** Bug 100039 has been marked as a duplicate of this bug. ***
[Bug c++/77875] C++ core issue 1288
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77875
--- Comment #5 from Jonathan Wakely ---
Yes, probably, but it doesn't seem useful for T{i} to do anything except bind a
reference of type T to i. Issue 1521 seems to be a problem with the wording,
such that it doesn't apply to references, but I doubt it will be resolved by
saying that T{i} does anything surprising. But maybe I'm missing something.
[Bug c++/77875] C++ core issue 1288
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77875
--- Comment #4 from Marek Polacek ---
Doesn't this depend on the resolution of Core 1521 (still "drafting"), dealing
with T{expr} where T is a reference type? Which is what this PR is about:
void
f ()
{
int i = 42;
using T = int&;
T t = T{i};
}
[Bug c++/77875] C++ core issue 1288
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77875
--- Comment #3 from Lénárd Szolnoki ---
A more worrisome example presumably for this same bug, it's a miscompilation:
template
decltype(auto) as_const(T& t) {
using const_ref = const T&;
return const_ref{t};
}
int main() {
int i = 42;
return as_const(i);
}
$ g++ -std=c++17 -O0 -fsanitize=undefined -Wall -pedantic main.cpp && ./a.out
main.cpp: In instantiation of 'decltype(auto) as_const(T&) [with T = int]':
main.cpp:9:22: required from here
main.cpp:4:23: warning: returning reference to temporary [-Wreturn-local-addr]
return const_ref{t};
^
main.cpp:4:23: runtime error: reference binding to null pointer of type 'const
int'
main.cpp:9:22: runtime error: load of null pointer of type 'const int'
bash: line 7: 802 Segmentation fault (core dumped) ./a.out
An UB is added where should be none.
[Bug c++/77875] C++ core issue 1288
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77875
Jonathan Wakely changed:
What|Removed |Added
Keywords||rejects-valid
Status|UNCONFIRMED |NEW
Last reconfirmed||2019-03-27
Ever confirmed|0 |1
--- Comment #2 from Jonathan Wakely ---
From PR 89857:
gcc doesn't compile the following well-formed program:
void foo () {
using T = int&;
int i{};
T{i};
}
$ g++ -std=c++17 -c gcc_bug2.cpp
gcc_bug2.cpp: In function ‘void foo()’:
gcc_bug2.cpp:4:8: error: invalid cast of an rvalue expression of type ‘int’ to
type ‘T’ {aka ‘int&’}
T{i};
Quoting from the standard draft:
http://eel.is/c++draft/dcl.init.list#3.9
Otherwise, if the initializer list has a single element of type E and either T
is not a reference type or its referenced type is reference-related to E, the
object or reference is initialized from that element (by copy-initialization
for copy-list-initialization, or by direct-initialization for
direct-list-initialization); if a narrowing conversion (see below) is required
to convert the element to T, the program is ill-formed.
According to the error, it looks like gcc skips this and tries to apply the
next rule:
Otherwise, if T is a reference type, a prvalue of the type referenced by T is
generated.
The prvalue initializes its result object by copy-list-initialization.
The prvalue is then used to direct-initialize the reference.
[ Note: As usual, the binding will fail and the program is ill-formed if the
reference type is an lvalue reference to a non-const type.
— end note ]
Note: The current behavior was originally a defect in the C++11 standard that
was corrected in CWG1288.
http://open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#1288
[Bug c++/77875] C++ core issue 1288
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77875 Jonathan Wakely changed: What|Removed |Added CC||leni536 at gmail dot com --- Comment #1 from Jonathan Wakely --- *** Bug 89857 has been marked as a duplicate of this bug. ***
