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[temp.local]/3 Wrong example? #1900

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timsong-cpp opened this issue Jan 18, 2018 · 1 comment
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[temp.local]/3 Wrong example? #1900

timsong-cpp opened this issue Jan 18, 2018 · 1 comment
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timsong-cpp commented Jan 18, 2018 •
edited 

template<class T, template<class> class U = T::template Base> struct Third { };
Third<Base<int> > t;                    // OK: default argument uses injected-class-name as a template

With T::template Base, lookup for Base in the scope of T aka Base<int> finds the injected-class-name, and this lookup does not ignore function names, so per CWG 1310 it should be considered to name the constructor of Base, making the code ill-formed.

If this analysis is correct, perhaps the example can be changed to

template<class T, template<class> class U = T::template Base> struct Third { };
Third<Derived<int> > t;                    // OK: default argument uses injected-class-name as a template
Third<Base<int> > t;                       // Error: injected-class-name in default argument names the constructor of Base ([class.qual])
@jensmaurer jensmaurer added the decision-required A decision of the editorial group (or the Project Editor) is required. label Feb 13, 2018
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MSVC rejects. ICC with different error messages. clang accepts with a warning.
Problem is T::template Base is a constructor name, even with intervening "template".
Editorial meeting consensus: Replace "Base" with "Derived", don't add a second line.

@jensmaurer jensmaurer removed the decision-required A decision of the editorial group (or the Project Editor) is required. label Mar 16, 2018
@jensmaurer jensmaurer mentioned this issue Mar 23, 2018
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@jensmaurer jensmaurer self-assigned this Mar 23, 2018
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