[Bug c++/105869] New: Use of this inside a lambda not inside a non-static method gives an interesting message

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105869

            Bug ID: 105869
           Summary: Use of this inside a lambda not inside a non-static
                    method gives an interesting message
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

Take:
```
auto t = []() { this; };
```

Currently GCC gives:
```
<source>:1:17: error: 'this' was not captured for this lambda function
    1 | auto t = []() { this; };
      |                 ^~~~
```
Except there is no this to capture and if you try to capture it, GCC gives:
```
<source>:1:11: error: invalid use of 'this' at top level
    1 | auto t = [this]() { this; };
      |           ^~~~
<source>: In lambda function:
<source>:1:21: error: 'this' was not captured for this lambda function
    1 | auto t = [this]() { this; };
      |                     ^~~~
```

Clang gives a clearer error message:
```
<source>:1:17: error: invalid use of 'this' outside of a non-static member
function
auto t = []() { this; };
                ^
```