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variant constructor and assignment shouldn't allow narrow conversion #463

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According to the standard, the constructor of a variant should construct a variant holding an alternative type T_i from an input of type T only if no narrow conversion is performed between T and T_i.

See documentation here: https://en.cppreference.com/w/cpp/utility/variant/variant
Constructor 4:

An overload F(T_i) is only considered if the declaration T_i x[] = { std::forward(t) }; is valid

As a direct consequence, the following code is valid (taken from the link above):

variant<float, long, double> z = 0; // OK, holds long
                                    // float and double are not candidates

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