Re: C++ expected-to-fail compilation goes through by not detecting mutable-specifier on lambda...

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On Mon, 2 Mar 2020 at 03:55, leon zadorin <leonleon77@gmail.com> wrote:
>
> On Mon, Mar 2, 2020 at 1:57 PM leon zadorin <leonleon77@gmail.com> wrote:
> >
> > template<typename T>
> > void foo(T const f)
> > {
> >     f();
> > }
> ...
> > the effect of 'mutable' (i.e. making operator()() non-const) are not
> detected by GCC/clang to a point where both of the following compile ok:
> > int main()
> > {
> >     foo([](){});
> >     foo([]() mutable {});
> > }
> >
>
> ... actually never mind, I take it back :)  I think GCC/clang are ok in
> that regard, reading https://en.cppreference.com/w/cpp/language/lambda
> "mutable: allows body to modify the parameters captured by copy, and to
> call their non-const member functions"
>
> makes it more clear for me... as in the above lambdas have no captures
> anyways (i.e. no effect), and with any captures (e.g. mutating with
> mutable) the expected compilation failure becomes evident, so in effect
> GCC/clang are doing this fine :)

What they're doing seems kinda OK, but I think it's not actually
allowed by the standard. The wording does say that unless the lambda
uses the mutable keyword, the function call operator is
const-qualified.

So I think this is a bug.