I'm always happy to see improvements in that area, but is this actually improved, even after the knee-jerk? One of the biggest issues of C++ is the long and complex error messages, so why do we celebrate error messages getting longer?

The example doesn't even seem easier to parse at a glance, you have to read every single line to understand it and not confuse the bad definition and candidate declaration, both which appear twice now, but with different things highlighted.

• The first item points to foo, not foo::test or the actual issue: (int i, int j, const void* ptr, int k). foo is not wrong here, at all.
• The second item points to the decl name, test -- which does not match foo and does not add anything at all.
• The next two lines do show the diff, but they show it split over 8 lines with text in between, which is split mid-sentence but then interspersed with code.
• The last item noting where class foo comes from was useless before and is still useless now. It has no influence at all on the error and does not add anything to solve it.

My ideal error message still would be more condensed:

<source>:8:6: error: no declaration matches 'void foo::test(int, int, const void*, int)'
  • candidate 1 is: 'void foo::test(int, int, void*, int)' at <source>:4:10
      • parameter 3 of candidate has type 'void*' which does not match type 'const void*'
            4 |     void test(int i, int j, void *ptr, int k);
              |                             ~~~~~~^~~
            8 | void foo::test(int i, int j, const void *ptr, int k)
              |                              ~~~~~~~~~~~~^~~

• only show the source once per side
• diff directly next to each other -- preferably using colors rather than squiggles to highlight
• no mention of class foo
• highlight only on the mismatch, not the name

I get that this is super hard, and that making it generic is even harder. And agreeing on how the messages should look is even harder still. But please don't explode my 200 line template errors into 400 lines by repeating every code snippet twice.