C.46: Recommend marking nonexplicit single-argument constructors with explicit(false)

[LocalSpook]

C.46 currently just says that nonexplicit single-argument constructors are rarely justified and suggests warning for any that aren't on some whitelist. Then the GSL section recommends making intent explicit with the nonstandard attribute [[implicit]].

Since C++20 introduced conditionally explicit specifiers, we can express this portably with explicit(false):

class Complex {
public:
    explicit(false) Complex(double d);   // OK: we want a conversion from d to {d, 0}
    // ...
};

Complex z = 10.7;   // unsurprising conversion

(I first saw this in P3094's sample implementation of basic_fixed_string.)

[hsutter]

Editors call: Thanks! We understand the motivation but we don't think making such declarations longer has significant benefits.