[Bug libstdc++/90389] New: std::deque::emplace tries to call wrong overload internally

[mikedlui+gccbugzilla at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90389

Bug ID: 90389
   Summary: std::deque::emplace tries to call wrong overload
internally
   Product: gcc
   Version: 9.1.0
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: libstdc++
  Assignee: unassigned at gcc dot gnu.org
  Reporter: mikedlui+gccbugzilla at gmail dot com
  Target Milestone: ---

Example: 

#include 
#include 

struct Foo {};
struct Bar {
Bar(Foo f1, Foo f2, size_t c) {}
};

int main() {
std::deque deq;
deq.emplace(deq.cbegin(), Foo{}, Foo{}, size_t{});
}

https://godbolt.org/z/Bqw5Qv

It looks like when instantiating `std::deque::emplace`, an incorrect overload
to:

 template 
template 
  void
  deque<_Tp, _Alloc>::
  _M_insert_aux(iterator __pos,
_ForwardIterator __first, _ForwardIterator __last,
size_type __n);

is chosen instead of the expected:

 template
  typename deque<_Tp, _Alloc>::iterator
  deque<_Tp, _Alloc>::
  _M_insert_aux(iterator __pos, _Args&&... __args);

[Bug boehm-gc/90390] New: incorrect list initialization behavior for references

[mikedlui+gccbugzilla at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90390

Bug ID: 90390
   Summary: incorrect list initialization behavior for references
   Product: gcc
   Version: 9.1.0
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: boehm-gc
  Assignee: unassigned at gcc dot gnu.org
  Reporter: mikedlui+gccbugzilla at gmail dot com
  Target Milestone: ---

https://stackoverflow.com/a/54120116/1371191

Example from the link:

int g_i = 10;
struct S {
operator int&(){ return g_i; }
};

int main() {
S s;
int& iref1 = s; // implicit conversion

int& iref2 = {s}; // clang++ error, g++ compiles fine:
  // `s` is converted
  // to a temporary int and binds with
  // lvalue reference

int&& iref3 = {s}; // clang++ compiles, g++ error:
   // cannot bind rvalue reference
   // to lvalue
}


Because `s` of type `S` is not reference-related to `int`, a temporary of type
`int` should be created during list initialization. Instead, it seems that the
user-defined conversion occurs first.

[Bug c++/90391] New: nonconforming value initialization when type T has a base class with a user-defined default constructor

[mikedlui+gccbugzilla at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90391

Bug ID: 90391
   Summary: nonconforming value initialization when type T has a
base class with a user-defined default constructor
   Product: gcc
   Version: 9.1.0
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c++
  Assignee: unassigned at gcc dot gnu.org
  Reporter: mikedlui+gccbugzilla at gmail dot com
  Target Milestone: ---

This is probably a low priority due to its edge-case nature, but here it is:

https://stackoverflow.com/q/54028846/1371191

#include 
struct A {
A(){}
int i;
};
struct B : public A {
int j;
};
int main() {
B b = {};
std::cout << b.i << b.j << std::endl;
}

Compiling:

$ g++ -std=c++11 -Wuninitialized -O2 a.cpp
a.cpp: In function ‘int main()’:
a.cpp:25:25: warning: ‘b.B::.A::i’ is used uninitialized in this
function [-Wuninitialized]
 std::cout << b.i << " " << b.j << std::endl


`b.B::.A::i` should be zero initialized (along with all of `b`) for
C++11, but it looks like it's getting initialized by A's default constructor
instead. This is the case for both C++11 and C++14.

As others mention in the stack overflow responses, this behavior is expected in
only expected in C++17/C++20, but for different reasons.