`modernize-use-trailing-return-type` might show invalid type in fix-it hint for lamdba with iterator

[firewave]

#include <algorithm>
#include <vector>

void f()
{
    auto it = [](const std::vector<int>& v) {
        return v.cbegin();
    };
}

<source>:6:15: warning: use a trailing return type for this lambda [modernize-use-trailing-return-type]
    6 |     auto it = [](const std::vector<int>& v) {
      |               ^
      |                                             -> const_iterator

https://godbolt.org/z/zzYMa4o5r

It should be suggesting std::vector<int>::const_iterator.

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: Oliver Stöneberg (firewave)

```cpp #include <algorithm> #include <vector>

void f()
{
auto it = [](const std::vector<int>& v) {
return v.cbegin();
};

auto it2 = [](const std::vector<int>& v) {
    return std::find_if(v.cbegin(), v.cend(), [](int i) -> bool {
        return i == 0;
    });
};

}

<source>:6:15: warning: use a trailing return type for this lambda [modernize-use-trailing-return-type]
6 | auto it = [](const std::vector<int>& v) {
| ^
| -> const_iterator
<source>:10:16: warning: use a trailing return type for this lambda [modernize-use-trailing-return-type]
10 | auto it2 = [](const std::vector<int>& v) {
| ^
| -> __gnu_cxx::__normal_iterator<const int *, std::vector<int>>

https://godbolt.org/z/zzYMa4o5r

It should be suggesting `std::vector<int>::const_iterator` in both cases.

The second case might also happen for other types.
</details>

[llvmbot]

@llvm/issue-subscribers-bug

Author: Oliver Stöneberg (firewave)

```cpp #include <algorithm> #include <vector>

void f()
{
auto it = [](const std::vector<int>& v) {
return v.cbegin();
};

auto it2 = [](const std::vector<int>& v) {
    return std::find_if(v.cbegin(), v.cend(), [](int i) -> bool {
        return i == 0;
    });
};

}

<source>:6:15: warning: use a trailing return type for this lambda [modernize-use-trailing-return-type]
6 | auto it = [](const std::vector<int>& v) {
| ^
| -> const_iterator
<source>:10:16: warning: use a trailing return type for this lambda [modernize-use-trailing-return-type]
10 | auto it2 = [](const std::vector<int>& v) {
| ^
| -> __gnu_cxx::__normal_iterator<const int *, std::vector<int>>

https://godbolt.org/z/zzYMa4o5r

It should be suggesting `std::vector<int>::const_iterator` in both cases.

The second case might also happen for other types.
</details>

[vbvictor]

Second case seems a bit hard.
I don's see the original sugared type preserved in the AST https://godbolt.org/z/farW8EqE3

[firewave]

I split up the ticket since the cases are not related.

[firewave]

The same is reported when using libc++: https://godbolt.org/z/TdnhssenY

[mizvekov]

This needs to recreate the nested name specifier for the return type taking the fix-it context into account.