[Bug c++/94025] Expected-to-fail compilation goes through by not detecting mutable-specifier on lambda

["daniel.kruegler at googlemail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94025

Daniel Krügler <daniel.kruegler at googlemail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |daniel.kruegler@googlemail.
                   |                            |com

--- Comment #1 from Daniel Krügler <daniel.kruegler at googlemail dot com> ---
In my opinion, this issue does not demonstrate a bug, but is based on an
incomplete analysis of what is going on here. 

1) It is correct, that the lambda function call operator is non-const in this
case. The result is that the function call operator of the lambda expression
will *not* be called in the shown example.

2) We have here a lambda expression without any capture. This means that the
standard requires the existence of an *additional* conversion function to a
pointer to function ([expr.prim.lambda.closure] p7 quoted from N4849). And
[expr.prim.lambda.closure] p11 says:

"The conversion function [..] is public, constexpr, non-virtual, non-explicit,
const, and has a non-throwing exception specification (14.5)."

So effectively a second function call resolution is in affect here, selecting
the conversion function (which is a const member function as specified above)
to function pointer as the only viable candidate (If both were viable, the
conversion function would be less preferred) via the surrogate call function
([over.call.object]). That explains IMO why the code is well-formed.

If you would try to mimic that with a user-defined class type, it would look
similar to the following one:

struct Lambda
{
  using f_t = void();
  f_t operator(); // "mutable"
  using fptr_t = f_t*;
  operator fptr_t() const;
};

Note that I use here the very rarely used syntax to declare (but not define) a
member function using a typedef for a function type to show the involved
function types more precisely.

The example would become invalid once you introduce a capture, because in this
case there would be no conversion function anymore.

I'm surprised that the Visual Studio compiler (I tested 2019) rejects the
original example, this looks like a bug to me, especially since that compiler
also handles the call resolution for the above defined Lambda type correctly. I
plan to report an issue for that compiler.