https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117294
--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #1)
> Even if we remove the constrained function template and just write an
> assertion based on the underlying built-in, there's no more information:
>
> struct A {
> explicit A(int) {}
> };
>
> template <typename T>
> struct Foo {
> T a = 1;
> };
>
> auto tryit() {
> static_assert( __is_constructible(Foo<A>) );
> }
For this simpler case, Clang gives:
def.cc:7:11: error: no viable conversion from 'int' to 'A'
7 | T a = 1;
| ^
def.cc:6:8: note: in instantiation of default member initializer 'Foo<A>::a'
requested here
6 | struct Foo {
| ^
def.cc:11:18: note: in evaluation of exception specification for 'Foo<A>::Foo'
needed here
11 | static_assert( __is_constructible(Foo<A>) );
| ^
def.cc:1:8: note: candidate constructor (the implicit copy constructor) not
viable: no known conversion from 'int' to 'const A &' for 1st argument
1 | struct A {
| ^
def.cc:1:8: note: candidate constructor (the implicit move constructor) not
viable: no known conversion from 'int' to 'A &&' for 1st argument
1 | struct A {
| ^
def.cc:2:14: note: explicit constructor is not a candidate
2 | explicit A(int) {}
| ^
1 error generated.
Which I suppose is much closer to what Barry is asking for. So it's certainly
possible, and I'm just not very imaginative.
