https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111419
Bug ID: 111419
Summary: Eager instantiation of function return type in concept
causes compilation error
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ldionne.2 at gmail dot com
Target Milestone: ---
The following code fails to compile on GCC:
------------------------------------------------------------
template <class F>
auto invoke(F f) -> decltype(f());
template <class F>
concept invocable = requires(F&& f) {
::invoke(f);
};
struct Incomplete;
template<class T> struct Holder { T t; };
static_assert(invocable<Holder<Incomplete>& ()>);
------------------------------------------------------------
It produces the following error on GCC:
<source>: In instantiation of 'struct Holder<Incomplete>':
<source>:6:11: required from here
<source>:10:37: error: 'Holder<T>::t' has incomplete type
10 | template<class T> struct Holder { T t; };
| ^
<source>:9:8: note: forward declaration of 'struct Incomplete'
9 | struct Incomplete;
| ^~~~~~~~~~
Compiler returned: 1
My understanding is that this should be valid, because nothing actually
requires instantiating the return type of `f()` here, but I'm not sufficiently
well-versed in the details of concepts to know for sure.
Clang and MSVC accept the code.
Godbolt: https://godbolt.org/z/9G5zj47an
