c++ - Is typename required or not here? -


consider code:

#include <memory>  template <class t, class deleter = std::default_delete<t>> class unique_ptr_wrapper: public std::unique_ptr<t, deleter> { public:     using typename std::unique_ptr<t, deleter>::unique_ptr;     operator t* () const {return this->get();} };  int main() {     unique_ptr_wrapper<int> upw{new int{42}}; }

g++5.1 compiles fine, although clang++ complains

error: typename allowed identifiers only

i agree don't have identifier here, typename not required. forbidden? compiler required @ least emit diagnostic?

edit code compiled fine without typename both g++ , clang++.

update seems g++ bug, reported here.

[class.inhctor]/p1, emphasis mine:

a using-declaration (7.3.3) names constructor implicitly declares set of inheriting constructors.

a constructor not type.

[temp.res]/p3-4:

3 when qualified-id intended refer type not member of current instantiation (14.6.2.1) , nested-name-specifier refers dependent type, shall prefixed keyword typename, forming typename-specifier. if qualified-id in typename-specifier not denote type, program ill-formed.

4 if specialization of template instantiated set of template-arguments such qualified-id prefixed typename not denote type, specialization ill-formed. usual qualified name lookup (3.4.3) used find qualified-id in presence of typename.

[class.qual]/p2:

in lookup in function names not ignored , nested-name-specifier nominates class c:

  • if name specified after nested-name-specifier, when looked in c, injected-class-name of c (clause 9), or

  • in using-declaration (7.3.3) member-declaration, if name specified after nested-name-specifier same identifier or simple-template-id’s template-name in last component of nested-name-specifier,

the name instead considered name constructor of class c.

applying "usual qualified name lookup" rules found in [class.qual], std::unique_ptr<t, deleter>::unique_ptr names constructor. not denote type. therefore, above quote [temp.res], program ill-formed (with diagnostic required).

in other words, appears gcc bug (though clang's error message use improvement too).


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