[Bug c++/77825] [7 Regression] return type deduction regression in C++17 mode

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77825

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |UNCONFIRMED
                 CC|                            |jason at gcc dot gnu.org
     Ever confirmed|1                           |0

--- Comment #2 from Jason Merrill <jason at gcc dot gnu.org> ---
This is due to the C++17 constexpr lambda change: previously, we would defer
instantiation of the lambda operator() until EOF, but now we instantiate it
immediately when we call it from y_combinator's operator().  Since we haven't
completed instantiating that function yet, it doesn't have a real return type
yet, so the mutual recursion leads to an error.