https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91243
Bug ID: 91243
Summary: is_invocable mishandles functions returning
indestructible types by value
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: rs2740 at gmail dot com
Target Milestone: ---
#include <type_traits>
struct S { friend struct F; protected: ~S(); };
struct F { S operator()() { return {}; } };
static_assert(!std::is_invocable_v<F>); // fires
The condition for is_invocable is
> The expression INVOKE(declval<Fn>(), declval<ArgTypes>()...) is well-formed
> when treated as an unevaluated operand
This expression is not well-formed when treated as a normal unevaluated
operand. It's only well-formed when it's the operand of decltype, due to the
special rule in [dcl.type.decltype]p2.
libc++ hard errors for this case. MSVC accepts the snippet above.
