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4270. Diagnose misuse of std::projected::operator*

Section: 24.3.6.4 [projected] Status: New Submitter: Jiang An Opened: 2025-05-30 Last modified: 2025-06-06

Priority: Not Prioritized

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Discussion:

std::projected::operator* is only intentionally used for type calculation like std::declval. Currently, if one attempts to call it in a potentially evaluated expression, the program is ill-formed, no diagnostic required because the operator* is not defined.

It seems better to diagnose such misuse, as we are already diagnosing misuse of std::declval (22.2.6 [declval]/2). Implementation used to perform such misuse via std::ranges::iter_move, but libc++ and libstdc++ have already switched not to do this (see microsoft/STL#5555).

Proposed resolution:

This wording is relative to N5008.

1. Modify 24.3.6.4 [projected] as indicated:

   namespace std {
     template<class I, class Proj>
     struct projected-impl { // exposition only
       struct type { // exposition only
         […]
         // models weakly_incrementable
         indirect_result_t<Proj&, I> operator*() const; // not defined
       };
     };
     […]
   }
   

   indirect_result_t<Proj&, I> operator*() const;
   

   -?- Mandates: This function is not odr-used (6.3 [basic.def.odr]).