[Bug c++/93705] [C++2a] Non-type literal class template-parameter types with mutable data members should not be allowed

[kevin at hart dot mn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93705

Kevin Hartman  changed:

   What|Removed |Added

 Status|UNCONFIRMED |RESOLVED
 Resolution|--- |INVALID

--- Comment #1 from Kevin Hartman  ---
I misread the spec. “Non-mutable” means “not marked with the ‘mutable’
specifier.

[Bug c++/93705] New: [C++2a] Non-type literal class template-parameter types with mutable data members should not be allowed

[kevin at hart dot mn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93705

Bug ID: 93705
   Summary: [C++2a] Non-type literal class template-parameter
types with mutable data members should not be allowed
   Product: gcc
   Version: 10.0
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c++
  Assignee: unassigned at gcc dot gnu.org
  Reporter: kevin at hart dot mn
  Target Milestone: ---

In the current implementation of C++2a, otherwise structural class types with
mutable data members are allowed as non-type template parameters.

The following sources seem to indicate that these should be rejected:
https://github.com/cplusplus/draft/blob/0354125f1cdfef949d80053528c4a1a4f74b1dea/source/templates.tex#L396
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1907r1.html

Perhaps non-const member functions should cause rejection as well? I couldn't
find anything about this.

Example:


struct Literal {
constexpr Literal() {}

// This is allowed (not const), but perhaps shouldn't be.
bool has_value = false;
};

template
void Test() {
// The below will fail if uncommented, but seems different
// from the language of the spec.

// error: assignment of member 'Literal::has_value' in read-only object
// lit.has_value = true; 
}

int main()
{
Test();
}