CWG3111 [temp.param] Correctly handle template parameter objects of array type #791

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1. t3nsor commented on Oct 30, 2025

   

   t3nsor

   on Oct 30, 2025

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   I find the proposed definition of "template parameter object" to be confusing: a definition that uses "with the property that" suggests that to determine whether something is a template parameter object, you check whether it has that property. This property should be in a separate sentence (as it currently is) to make it clear that it's not part of the definition.
2. katzdm commented on Oct 30, 2025

   

   katzdm

   on Oct 30, 2025

   Last edited by katzdm

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   I find the proposed definition of "template parameter object" to be confusing: a definition that uses "with the property that" suggests that to determine whether something is a template parameter object, you check whether it has that property. This property should be in a separate sentence (as it currently is) to make it clear that it's not part of the definition.

   Yeah, I hear that. I'm trying to separate the definition from the template parameter mechanism, since we evidently now have template parameter objects that are never consequent from an instantiation. On the other hand, their only prior definition was "the static storage duration objects that happen to be consequent from an instantiation." And yet, that property (i.e., unique under template-argument-equivalence) is their most important quality.

   So I think the the confusion here is that "template parameter objects" is more correctly a "family" or "class" of not-very-special static storage duration objects. Here's another try:

   Updated suggestion resolution

   Modify [temp.param]/13 as follows:

   There is a (possibly empty) set of static storage duration objects of non-volatile const type, called template parameter objects, for which no two such objects are template-argument-equivalent ([temp.type]). An id-expression naming a constant template parameter of class type T denotes a static storage duration object of type const T, known as a template parameter object, the template parameter object which is template-argument-equivalent ([temp.type]) to the corresponding template argument after it has been converted to the type of the template parameter ([temp.arg.nontype]). No two template parameter objects are template-argument-equivalent.

   Add the following note after [temp.param]/13:

   [ Note: There can be template parameter objects of array type ([meta.reflection.array]), but such objects are never denoted by an id-expression that names a constant template parameter. — end note ]

   Modify the specification of [meta.reflection.queries]/7 as follows:

   Effects: Equivalent to:

     if constexpr (is_annotation(R)) {
       return C;
   + } else if constexpr (is_array_type(type_of(R)) {
   +   return reflect_constant_array([:R:]);
     } else {
       return reflect_constant([:R:]);
     }
3. katzdm commented on Oct 31, 2025

   

   katzdm

   on Oct 31, 2025

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   Tomasz points out the issue also exists for function-to-pointer conversion:

   void f() {}
   
   constexpr auto *fptr = &f;
   constexpr auto &fref = f;
   
   static_assert(constant_of(^^fptr) == constant_of(^^fref));  // currently holds

   Consider also the following:

   template <auto &F> void tfn() {}
   void f() {}
   
   constexpr auto &fref = f;
   static_assert(constant_of(^^fref) == template_arguments_of(^^tfn<f>)[0]);  // currently fails

   This motivates the following updated change for [meta.reflection.queries]/7:

   Effects: Equivalent to:

     if constexpr (is_annotation(R)) {
       return C;
   + } else if constexpr (is_array_type(type_of(R)) {
   +   return reflect_constant_array([:R:]);
   + } else if constexpr (is_function_type(type_of(R)) {
   +   return reflect_function([:R:]);
     } else {
       return reflect_constant([:R:]);
     }
4. jensmaurer commented on Nov 2, 2025

   

   jensmaurer

   on Nov 2, 2025

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   CWG3111
5. jensmaurer

   changed the title [-][temp.param] Correctly handle template parameter objects of array type[/-] [+]CWG3111 [temp.param] Correctly handle template parameter objects of array type[/+] on Nov 2, 2025
6. tkoeppe

   closed this as completedin draft#8485on Dec 13, 2025