[BUG] Can't declare idiomatic `swap`

[JohelEGP]

Title: Can't declare idiomatic swap.

Description:

See https://github.com/hsutter/cppfront/issues/345#issuecomment-1585417506. There's code to make swap noexcept by default, but that can only be in or move, and not inout as required by swap.

Minimal reproducer (https://cpp2.godbolt.org/z/xsx5eEfWY):

t: type = {
  swap: (inout this, inout that) = { }
}
main: () = { }

Commands:

```bash cppfront main.cpp2 clang++17 -std=c++23 -stdlib=libc++ -lc++abi -pedantic-errors -Wall -Wextra -Wconversion -I . main.cpp ```

Expected result:

An idiomatic swap.

Actual result and error:

Output:

main.cpp2(2,32): error: a 'that' parameter must be in or move

[ntrel]

I started working on this, but I haven't figured out yet why calling swap generates the CPP2_UFCS macro instead of calling the member function. https://github.com/hsutter/cppfront/compare/main...ntrel:cppfront:inout-that

[JohelEGP]

That's because all calls of the form expression.function(expression-list arguments) are candidates for UFCS.

[ntrel]

But swap is declared as a member function, so it shouldn't try UFCS. Probably somewhere else needs updating in the compiler.

[JohelEGP]

No. cppfront doesn't implement lookup, so it has to defer to the lowered Cpp1 code to decide it wants to use the member function. It does that through the UFCS macro.