report `modernize-use-trailing-return-type` for lambdas

[firewave]

void f()
{
    auto f = []() /*-> bool*/ {
        return false;
    };
}

Currently no warning is generated for the code above.

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: Oliver Stöneberg (firewave)

```cpp void f() { auto f = []() /*-> bool*/ { return false; }; } ```

Currently no warning is generated for the code above.

[firewave]

I would be great if this was opt-in and the existing checks were also opt-out as I would only want to apply this change to the lambdas in my code bases.

[vbvictor]

Funnily enough, there is actually a warning on trunk with your exact code. https://godbolt.org/z/b96jc4Por
But only in c++11 mode.

[firewave]

But only in c++11 mode.

In our case the analysis is run in C++11 mode.

We are using run-clang-tidy to run the analysis maybe that is not handling -std properly. We also have some files which are build in C++17 mode so that might also contribute to it. Will investigate...

[firewave]

Stupid me. We obviously had the check disabled because we do not want to use the trailing type on regular functions...