[Bug c++/86147] New: Lambda is capturing a non-ODR-used constexpr

[julian.panetta at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86147

            Bug ID: 86147
           Summary: Lambda is capturing a non-ODR-used constexpr
           Product: gcc
           Version: 8.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: julian.panetta at gmail dot com
  Target Milestone: ---

Minimal example:

#include<array>
struct A { static constexpr size_t value = 0; };

template<class C>
void foo() {
    constexpr size_t N = C::value;
    auto f = [&]{  std::array<int, N> a; };
};

int main() { 
    foo<A>();
    return 0;
}

On gcc 8.0.1 and 8.1.1 (built from the latest revision on SVN), this code
produces an error:

error: use of ‘this’ in a constant expression
     auto f = [&]{  std::array<int, N> a; };
                                       ^

Apparently N is captured despite it not being ODR-used, leading to the error.
The error is not reported on gcc 7, and it strangely disappears if C::value is
replaced with a literal.