https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85965
Bug ID: 85965
Summary: G++ gives cryptic error instead of incomplete type
Product: gcc
Version: 8.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: psmith at gnu dot org
Target Milestone: ---
Compiling code with GCC 8.1 / binutils 2.30 (built locally on GNU/Linux amd64)
which previously compiled and worked OK with GCC 6.2 and 7.3.
I received a very cryptic error that had me running around reworking class
implementations for quite a while before I realized the problem: I had an
incomplete type. I don't know if there's anything G++ could do better here,
but FYI I had this code:
class Bar
{
public:
struct Less
{
bool operator()(const Bar& lhs, const Bar& rhs) const;
bool operator()(const Bar* lhs, const Bar* rhs) const;
};
};
class Biz;
#include <set>
class Foo
{
std::set<const Biz*, Bar::Less> _map;
};
It's not immediately clear that the incomplete Biz class is a problem,
especially in my code which is significantly more complex and crosses multiple
header files, and G++ doesn't give a very helpful (to me) error:
$ g++ -o set.o -c set.cpp
In file included from x86_64-generic-linux-gnu/include/c++/8.1.0/set:60,
from set.cpp:13:
x86_64-generic-linux-gnu/include/c++/8.1.0/bits/stl_tree.h: In instantiation of
'class std::_Rb_tree<const Biz*, const Biz*, std::_Identity<const Biz*>,
Bar::Less, std::allocator<const Biz*> >':
x86_64-generic-linux-gnu/include/c++/8.1.0/bits/stl_set.h:133:17: required
from 'class std::set<const Biz*, Bar::Less>'
set.cpp:17:37: required from here
x86_64-generic-linux-gnu/include/c++/8.1.0/bits/stl_tree.h:452:21: error:
static assertion failed: comparison object must be invocable with two arguments
of key type
static_assert(__is_invocable<_Compare&, const _Key&, const _Key&>{},
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If I had included the complete type for class Biz, the compiler would have seen
that Biz is a subclass of Bar and it would have been fine; adding in the header
file fixed my problem:
class Bar
{
public:
struct Less
{
bool operator()(const Bar& lhs, const Bar& rhs) const;
bool operator()(const Bar* lhs, const Bar* rhs) const;
};
};
class Biz : public Bar
{}
#include <set>
class Foo
{
std::set<const Biz*, Bar::Less> _map;
};
