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Subject: Re: CTADs and equivalent parameter list
From: sdkrystian (sdkrystian_at_[hidden])
Date: 2019-08-04 09:13:54


See http://eel.is/c++draft/temp.over.link#7.sentence-1Sent from my Samsung Galaxy smartphone.
-------- Original message --------From: Tobias Loew via Std-Discussion <std-discussion_at_[hidden]> Date: 8/4/19 05:39 (GMT-05:00) To: std-discussion_at_[hidden] Cc: tobi_at_[hidden] Subject: [std-discussion] CTADs and equivalent parameter list Hi,I think the wording of [temp.deduct.guide] (3) leaves room for interpretation. It says"... Two deduction guide declarations in the same translation unit for the same class template shall not have equivalent parameter-declaration-clauses."I couldn't find any definition of "equivalence" between parameter-declaration-clauses and especially none for parameter-declaration-clauses containing template-types.So, for example, the following code may or may not be valid. #include <type_traits> template<typename T>class A{public:    A(int){}    A(double){}};template<typename T, typename = std::enable_if_t<std::is_integral_v<T>>>A(T) -> A<int>;template<typename T, typename = std::enable_if_t<std::is_floating_point_v<T>>>A(T) -> A<double>;                    // <- equivalent parameter-declaration-clause as aboveint main(){    auto a = A{5};    auto b = A{5.123};} Apparently, all major compiler's latest versions agree that the code above is valid, though MSVC 19.20 and earlier rejected it and accepted only the following *single CTAD* version:template<typename T, typename = std::enable_if_t<std::is_integral_v<T> || std::is_floating_point_v<T>>>A(T) -> A<typename std::conditional_t<std::is_integral_v<T>, int, double>>;I think, the last sentence of [temp.deduct.guide] (3) should be more specific, e.g.:"... Two deduction guide declarations in the same translation unit for the same class template shall not have equivalent parameter-declaration-clauses <new>for a given specialization</new>."Tobias



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