https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105553
Bug ID: 105553
Summary: Deduction when attempting to create an array with an
element type that is an abstract class
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: glenjofe at gmail dot com
Target Milestone: ---
Starting with GCC11, the following definition of a trait to check if a type is
abstract no longer works in any standard mode:
template<class T>
class is_abstract {
template<class U>
static char (&check(U(*)[1]))[2];
template<class U>
static char check(...);
public:
static const bool value = sizeof(check<T>(0)) == sizeof(char);
};
struct abstract {
virtual void function() = 0;
};
static_assert(is_abstract<abstract>::value, "ok");
The static assertion trigger on GCC11 and above, even in older standard modes.
It compiles fine on GCC10 and below.
Various libraries in Boost use the above definition of is_abstract especially
in C++03. The basis was the following wording in [temp.deduct] until C++17:
"Type deduction may fail for the following reasons:
- Attempting to create an array with an element type that is void, a function
type, a reference type, or an abstract class type, or attempting to create an
array with a size that is zero or negative."
The "or an abstract class type" was absent in C++03, added by CWG337.
