Consider the following snippet:

    template <int>
    struct bar {};

    template <class I>
    auto foo(I i) -> bar<i()> { return {}; }

    int main()
    {
        foo([]{ return 1; }); // (0)
    }

This compiles and works as intended on clang++5, but produces a compile-time error on g++7:

    prog.cc:5:25: error: template argument 1 is invalid
    auto foo(I i) -> bar<i()> { return {}; }
                            ^
Live example:
https://wandbox.org/permlink/6cbnRiTyphIaf2yh

--

I think this is a bug because:

* `[]{ return 1; }` is implicitly `constexpr`, as `1` is a "core constant expression".

* In the context of the function call marked with "(0)", the argument `i` is a closure generated by the lambda expression `[]{ return 1; }`. The closure has a `constexpr operator()` overload which returns a `1` (a "core constant expression").

* `bar<i()>` is a valid "trailing return type", as `i()` can be a "core constant expression" if it results in a `constexpr` function invocation.

---

Additional info, context and examples:
http://stackoverflow.com/questions/43043397