[Bug c++/97640] New: GCC doesn't optimize-out outside-affecting lambdas within y-combinator while clang does.

[Bug c++/97640] New: GCC doesn't optimize-out outside-affecting lambdas within y-combinator while clang does.

[hotguest1 at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97640

            Bug ID: 97640
           Summary: GCC doesn't optimize-out outside-affecting lambdas
                    within y-combinator while clang does.
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hotguest1 at hotmail dot com
  Target Milestone: ---

Suppose we have a y-combinator, so that we can write lambdas in recursive
fashion.

    template <class F>
    struct y_combinator {
        F f;
        template <class... Ts>
        constexpr decltype(auto) operator()(Ts&&... ts) const { return
f(std::ref(*this), std::forward<Ts>(ts)...); }

        template <class... Ts>
        constexpr decltype(auto) operator()(Ts&&... ts) { return
f(std::ref(*this), std::forward<Ts>(ts)...); }
    };


Let's have a constexpr-qualified lambda

    auto sum = y_combinator{[&](auto self,int a,int b,int res = 0) mutable {
        if (a>b) {
          return res;
        }
        res += a;
        return self(a+1,b,res);
    }};

This lambda is constexpr-qualified.
So both gcc and clang do optimize-out all the things in the name of constexpr
expansion.
https://compiler-explorer.com/z/qx4MGE

==============================================================================

Let's try an outside-affecting lambda

    auto sum = y_combinator{[&,res=0](auto self,int a,int b) mutable {
        if (a>b) {
          return res;
        }
        res += a;
        return self(a+1,b);
    }};

This non-constexpr-qualified lambda is optimized-out in clang but in gcc it is
not.
https://compiler-explorer.com/z/hPGWod

[Bug c++/97640] GCC doesn't optimize-out outside-affecting lambdas within y-combinator while clang does.

[hotguest1 at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97640

--- Comment #1 from sandthorn <hotguest1 at hotmail dot com> ---
The y-combinator implementation is from Barry's and Mathemagician's answer on
SO.
https://stackoverflow.com/a/40873657/6370128