[Bug c++/102157] New: floating-integral conversions is not permitted in the user-defined conversion sequence in converted constant expression

["xmh970252187 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102157

            Bug ID: 102157
           Summary: floating-integral conversions is not permitted in the
                    user-defined conversion sequence in converted constant
                    expression
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: xmh970252187 at gmail dot com
  Target Milestone: ---

struct A{
    constexpr A(double const& rf){}
};
template<class T, T B>
struct C{};
int main() {
    C<A,0> b;
}

This code is accepted by GCC while rejected by Clang.  

[temp.arg.nontype] p2 
> A template-argument for a non-type template-parameter shall be a converted constant expression ([expr.const]) of the type of the template-parameter.  

>From a prvalue of type `int` to type `A`, the user-defined conversion sequence
will involve floating-integral conversions that are not permitted in
[expr.const] p10. GCC should reject this example.