You already can:

void foo(double arg1, std::same_as<bool> auto arg2)...

On Fri, Jan 9, 2026, 18:18 David Brown via Std-Proposals <
std-proposals_at_[hidden]> wrote:

>
>
> On 09/01/2026 17:37, Steve Weinrich via Std-Proposals wrote:
> > Consider two methods:
> >
> > void interest (double arg1, bool minor);
> >
> > void interest (double arg1, int factor);
> >
> > The call: interest(5.5, 5); is ambiguous because 5 will freely convert
> > to a bool.
>
> No, it is not ambiguous - the best fit overload will be used. The rules
> for overload resolution are a bit complicated, but in this case the
> results are clear.
>
> <https://en.cppreference.com/w/cpp/language/overload_resolution.html>
>
> >
> > I was wondering what y’all would think of narrowing this behavior by
> > this addition:
> >
> > void interest (double arg1, *explicit* bool minor);
> >
> > Potentially, this could be applied to all arguments:
> >
> > void *explicit* interest (double arg1, bool minor);
> >
>
> To me, a bigger issue is when you /don't/ have overloads :
>
>
> void interest (double arg1, bool minor);
>
> In this situation, the implicit conversion of an int to a bool for
> "interest(5.5, 5)" is likely to be an error on the programmer's side,
> but it is accepted without question by C++ compilers. To avoid this,
> some people use "strong bool" types that have much stricter implicit
> conversions, such as :
>
> class Bool {
> bool v;
> public :
> constexpr Bool(bool v) noexcept : v(v) {}
> Bool(auto v) = delete;
> explicit constexpr operator bool() const noexcept { return v; }
> };
>
> Then you can declare the function :
>
> void interest (double arg1, Bool minor);
>
> and use it with a bool type as the second argument, but not with another
> type that has an implicit conversion to bool.
>
> An alternative way to block non-bool calls would be to write :
>
> void interest (double arg1, bool minor);
> void interest (double arg1, auto) = delete;
>
>
> But it would be very nice (IMHO) if you could just write :
>
> void interest (double arg1, explicit bool minor);
>
> and get the same effect (that is, a function candidate would not be
> considered "viable" in overload resolution if the second argument
> required an implicit conversion sequence to the specified type).
>
> --
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>