https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97841
Bug ID: 97841
Summary: [C++17] is_invocable handling of incomplete return
type is wrong
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: jason at gcc dot gnu.org
Target Milestone: ---
I already mentioned this to Jonathan directly, but thought I should probably
also put it in bugzilla.
is_invocable and invoke_result condition their result on whether
"The expression INVOKE(declval<Fn>(),declval<ArgTypes>()...) is well-formed
when treated as an unevaluated operand"
It seems that we currently test for this by checking whether decltype of the
above is well-formed, but that seems wrong to me, since decltype of a call of
incomplete type is well-formed, but that call in any other unevaluated context
is ill-formed. So we accept this testcase, but VC++ and clang/libc++ reject
it:
#include <type_traits>
struct A;
using fn = A(*)(int);
static_assert (std::is_invocable_v<fn,int>); // error, A is incomplete
Various other places in the library talk about an expression being "well-formed
when treated as an unevaluated operand" and could probably use a check to make
sure they don't have the same problem.
I imagine that changing decltype(INVOKE(...)) to decltype(0,INVOKE(...)) in
various places would be a simple fix?
