Fix clang compilation when T is constructible from other types #64
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The problem was with copy/move constructors of boost::optional<T>, which invoked optional_base<T> constructors with a single argument. Since optional_base has copy/move constructors along with initializing constructors taking a single argument, the latter may be considered by the compiler for viability. While doing so, the compiler may instantiate the template constructor of T with an argument of optional_base<T>, which in turn may fail if the constructor attempts to inspect the type of its argument (e.g. to constrain the set of acceptable types). Specifically, this happens with clang in C++03 mode, when boost::multiprecision::number is wrapped in boost::optional and a copy constructor of boost::optional is invoked. This commit fixes the problem by destinguishing copy/move constructors of optional_base from initializing constructors with an additional tag argument.
… invoke T's template constructors.
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Reference to the problem with Boost.Multiprecision: boostorg/integer#11 (comment). |
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The problem was with copy/move constructors of
boost::optional<T>, which invokedoptional_base<T>constructors with a single argument. Sinceoptional_basehas copy/move constructors along with initializing constructors taking a single argument, the latter may be considered by the compiler for viability. While doing so, the compiler may instantiate the template constructor ofTwith an argument ofoptional_base<T>, which in turn may fail if the constructor attempts to inspect the type of its argument (e.g. to constrain the set of acceptable types). Specifically, this happens with clang in C++03 mode, whenboost::multiprecision::numberis wrapped inboost::optionaland a copy constructor ofboost::optionalis invoked.This PR fixes the problem by destinguishing copy/move constructors of
optional_basefrom initializing constructors with an additional tag argument. The PR also adds a test.