[llvm-bugs] [Bug 40988] New: decltype resolution doesn't terminate recursion

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https://bugs.llvm.org/show_bug.cgi?id=40988

            Bug ID: 40988
           Summary: decltype resolution doesn't terminate recursion
           Product: clang
           Version: 8.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++11
          Assignee: unassignedclangbugs at nondot.org
          Reporter: Simon.Richter at hogyros.de
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

GCC exhibits the same bug, see
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89563

I'm trying to build a commutative operator+ for a class, as a freestanding
template operator+ that accepts my class on the right hand side and reverses
the argument order:

    struct one {};

    struct two
    {
        two() { }
        two(one const &) { }
        operator one() const { return one{}; }
        two operator+(one const &) const { return two{}; }
    };

    two operator+(two const &, two const &) { return two{}; }

    template<typename T>
    auto operator+(T const &lhs, two const &rhs) -> decltype(rhs + lhs)
    {
        return rhs + lhs;
    }

    void test()
    {
        one o;
        two t;
        auto a = o + t;
    }

This fails to compile, because the decltype for the return type is resolved
recursively through the template, even though a nontemplate function exists.
This remains the case even if I make a freestanding operator+(two const &, one
const &); 

icc and MSVC compile this code correctly.

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