[Bug c++/87377] New: error with generic lambda accessing static field through argument within return type

[comexk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87377

            Bug ID: 87377
           Summary: error with generic lambda accessing static field
                    through argument within return type
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: comexk at gmail dot com
  Target Milestone: ---

…that's a mouthful.  Here's the code:

template <auto _val>
struct const_val {
    static constexpr auto val = _val;
};
int main() {
    auto lambda = [](auto i) -> const_val<i.val> { while (1); };
    lambda(const_val<5>());
}

Currently, GCC trunk rejects this:

% /tmp/gcct/bin/g++ b.cpp -std=c++2a
b.cpp: In function ‘int main()’:
b.cpp:6:48: error: template argument 1 is invalid
6 |     auto lambda = [](auto i) -> const_val<i.val> { while (1); };
  |      

However, Clang accepts it.

I think this is a bug in GCC, because GCC does accept i.val as a constant
expression within the function body; for example, the example compiles if the
definition of 'lambda' is changed to:

    auto lambda = [](auto i) {
        const_val<i.val> c;
        while (1);
    };

It also accepts this:

    auto lambda = [](auto i) -> const_val<decltype(i)::val> { while (1); };