`-Wdeprecated-this-capture` and `-Wunused-lambda-capture` contradict each other

[r-barnes]

I've been unable to build an MWE for this issue, but in our (large, complex) codebase I observe that it's possible to have -Wdeprecated-this-capture trigger to indicate that we need

[=, this]() { lambda }

but once this has been added -Wunused-lambda-capture will flag this as being unused.

In one instance of the above situation this was needed to make a call to another method of the function. It was possible to mark that method as static after which this was no longer required, so perhaps there are situations where ``Wdeprecated-this-capturethe compiler determines thatthis` is needed, but `-Wunused-lambda-capture` is able to do a deeper analysis and determine it is not?

I'll continue to look for an MWE, but wanted to flag this in case someone who sees it has ideas.

[efriedma-quic]

Both warnings can be correct. Consider something like the following:

struct A {
    void f(int*);
    static void f(double*);
    template <typename T>
    void g() {
        [=,this]() { f(T()); }();
    }
};

void z() { A{}.g<double*>(); }
void zz() { A{}.g<int*>(); }

[r-barnes]

@efriedma-quic - Thanks for looking into this! I thought it might be something like that.

This is problematic, though, because the language is deprecating the implicit this which will force users to specify this and there's no way to mark it as maybe_unused. The result is that it will become impossible to use -Wunused-lambda-capture on a large codebase.

A rough workaround could be to have -Wunused-lambda-capture ignore this. Perhaps there's some better way I'm not thinking of?