https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102199
Bug ID: 102199
Summary: is_default_constructible incorrect for an inner type
with NSDMI
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: eyalroz1 at gmx dot com
Target Milestone: ---
Stackoverflow discussion: https://stackoverflow.com/q/69050558/1593077
Related LLVM bug: https://bugs.llvm.org/show_bug.cgi?id=38374
GodBolt: https://godbolt.org/z/snPf7Ks4W
Consider the following program:
#include <type_traits>
struct outer {
struct inner {
// inner() { }
unsigned int x = 0;
};
// static_assert(std::is_default_constructible<inner>::value,
// "not default ctorable - inside");
};
static_assert(std::is_default_constructible<outer::inner>::value,
"not default ctorable - outside");
It compiles. But if we uncomment the first static_assert - it evaluates to
false. Mind you: Not because struct inner is incomplete; it is simply deemed to
not be default-constructible. But - it _is_ default constructible. And if we
add a method to struct outer which default-constructs an inner, it will work.
Also note that if we uncomment the explicit default ctor the definition of
struct inner, both asserts pass.
clang++ seems to exhibit this too (also with -stdlib=libc++). I'm not sure
whether this is an actual bug in the library, or whether the standard mandates
this in some freakish way, but - it's just wrong.
